Thank you for all this! I've been perusing it for a few days now as its given me quite a lot to think about. I will be extremely curious on more of your thoughts after I post more of the 1866 world in review section, mostly because the route that will be taken is, in essence, the same, but will have two Pacific terminus for Russia that they consider!
Thank you and sorry if I went a bit overboard with my suggestions. If it is not off topic here are a couple of further thoughts (if it is offtopic, please just say so and I will stop):
1. The privately owned Ural Railway mentioned in my previous post (first going from Perm to Yekaterinburg built in 1875-1878 and then from there to Tyumen built 5 years later) is extremely important as it connects Kama-Volga basin in European Russia and Ob-Irtysh basin encompassing basically whole Western Siberia.
If ITTL it is build a few years earlier it could potentially lead to massive consequences in regards to China. IOTL in 1862-1878 there was massive Dungan Revolt and Qing government among other things completely lost control over Xinjiang.
Most Xinjiang was ruled by Yakub Beg (and his state Yettishar was even recognized by several states including Ottoman Empire).
Russia in 1871 invaded Xinjiang and occupied a substantial part of it (Ili Valley around Kulja aka Yining). While in theory it was done in Qing name, Russia was extremely reluctant to return territory to China even after the governmental army defeated rebels in 1875-1878. This lead to so called Kulja Crisis and eventually Russia backed down (although retaining around 20% of occupied territory as well as getting monetary compensation and trade concession from China).
If ITTL the railroad is completed by 1878 or better yet by 1875, it could potentially make Russian position way more hawkish, perhaps even leading to a border war with China.
IOTL Russia could project very little power into Xinjiang as it lies far from Russia territory. But having a railway connecting Volga and Ob-Irtysh basin could potentially improve Russian logistics quite a bit as it is possible to sail up Irtysh River all the way to Chinese border in Northern Xinjiang (Russia by 1870s had a substantial steamer fleet in Ob-Irtysh basin and of course in Volga basin, but moving men and cargos from one to the other was extremely costly before the railroad was completed).
Though it is still a few hundred kilometers from territory occupied IOTL it still makes Russian logistics and thus military position in the region. Moreover, Russia could potentially increase its zone of control in Northern Xinjiang to include the Chinese portion of upper Irtysh basin and the territory between it and Ili Valley basically encompassing north-western half of Dzungaria (if the railroad is built in early 1870s Russia can possibly even conquer Yakub Beg’s state before China, but this seems a bit too early and too smooth for Russia).
This greatly strengthens Russian position in the region thus making potential conflict with Qing a lot less dangerous. In the Far East Russia had a relatively modern and large Siberian Military Squadron based in Vladivostok and an ability to bring there reinforcements from overseas (as well as population in the border regions comparable to Chinese population in Northern Manchuria and thus an ability to beat local Chinese forces with the help of far superior military technology and organization; China bringing a massive army from China Proper is logistical nightmare).
All this together probably makes Russia a lot more adventurous in regards to China. If China can swallow the bitter pill and recognize the Russian rule over North-Western Dzungaria or if this leads to a war between Russia and China is anybody’s guess. But if the war happens, I don’t think China has a good chance defeating European power in 1870s and thus such a war probably ends in another treaty of humiliation for China with China potentially losing large part if not all Xinjiang and probably also northern Manchuria aka Heilongjiang and Jilin provinces (where unlike Xinjiang there is a substantial Han population, but at most 600-700 thousand i. e. equal to Russian population of bordering regions thus making the region integration into Russia possible)
2. IOTL there was a fierce debate what railways to build in the region and in what order. Three main options were Ural Railway mentioned above and two variants of the railway connecting Ural and Siberia to the main Russian railway network: northern (going from Vologda to Vyatka and then to Perm) and southern (going from Nizhny Novgorod to Kazan and from there to Yekaterinburg and Tyumen).
IOTL it was decided to first build Ural Railway (connecting main factory districts of northern Ural) that was built in 1875-1878 and in 1875 Alexander II signed the order to start building the southern variant of Siberian railway. The first one was built but the second one got delayed because of emerging crisis with Ottoman Empire (and the related massive military spending). When Russia was ready to get back to building railway to Ural and Siberia in early 1880s it was decided to first build the most crucial section of the railway connecting Yekaterinburg and Tyumen (which was completed in 1882-1885). But by that time the situation has changed since a new option emerged further south (were a railway to Samara and crucially a bridge over Volga was built in the meantime) and thus the initially agreed upon railway never fully emerged.
If IOTL Russia is in a better financial situation and/or has more investment from abroad (as I think is currently alluded; but we have to wait till 1866 world in review is posted as
@EnglishCanuck suggests) years long debates of what is more important connecting Ural factories together or connecting European Russia to Siberia can potentially be avoided. If Russia can afford both options, the construction of both railways can begin at least in early 1870s thus making Perm-Yekaterinburg-Tyumen railroad a reality by 1875.
Section further west may take a bit more time (though if the money is there it can be built quickly as it close to Volga and Kama rivers and thus the railway may be built from several points simultaneously and men and supplies can be easily transported wherever necessary).
So if Russia has a bit more money, then Perm-Yekaterinburg-Tyumen railroad can be built by mid 1870s with the connecting railway following when the opportunity presents itself. However, if Russia has substantially more money available, the section from Nizhny Novgorod to Kazan and from there either to directly to Yekaterinburg or to Perm can be built simultaneously (second option is cheaper as it is ~250 km shorter and requires less bridges, second option shortens the distance to Siberia by ~150 km). So western Siberian navigable river system can potentially be connected to Moscow and St. Petersburg by railway by mid-1870s, thus making logistics in Siberia a lot cheaper and easier.
3. If Russia completes Perm-Yekaterinburg-Tyumen railway by 1870s it would dramatically increase the settlement of Western Siberia. IOTL in 1888-1894 that is after this railway was completed and became fully operational and before the western section of the Trans-Siberian railway was completed on average there were 65 thousand settlers per year into Siberia peaking at 92 thousand in 1892 (compared to 200 thousand settlers a year on average traveling by Trans-Siberian in 1897-1917 peaking at 665 thousand in 1908 and compared to less than 10 thousand per year on average for 1861-1880). While not all these settlers used Perm-Tyumen railway, most did (in 1888-1891 70-80% settlers to Siberia passed through Tyumen, in 1892-1894 85-90% did).
Thus the completion of the Perm-Tyumen railway while on average brings 3 times less settlers to Siberia than completed the Trans-Siberian railway (on peak the difference is larger, but in 1908 when 665 thousand settlers arrived to Siberia was also the peak of agrarian reform and massive governmental program to encourage the resettlement), it is at least 20 times more than resettled to Siberia before this railroad was completed.
Thus, if Perm-Tyumen railroad is finished in 1875 by 1885 the population of Siberia should increase by at least 500 thousand people compared to IOTL (assuming the first 5 years of railroad operating would bring 30 thousand additional settlers and from the 6th year onwards it would bring 65 thousand additional settlers). To compare in 1885 the total population of Siberia and Steppes District roughly corresponding to modern Kazakhstan was 5902 thousand people).
This alone could potentially bring important changes to the demography over-the-Urals part of Russian Empire.
Of course the railroad connecting the Ural Railway to the other Russian railways can potentially increase this stream of settlers (perhaps to 100-125 thousand per year) and continuing this road further at least to Tomsk would increase the stream of settlers to close OTL numbers for 1897-1917.