But the Lake Baikal area is decent land, perfectly worthwhile. The Chinese may not have the means to do it, but the Manchus, Mongols did.
Manchuria and Mongolia are not so different from Siberia in climate. The Manchu led Qing empire therefore did posses the technological ability to colonize. I also disagree that the Siberian rivers would prove disadvantages, as they all run from south to north, making colonization from the south easier than from the west. As far as fur goes, the Chinese were big buyers of Russian fur. Many Russian Siberian towns existed only to trade with the Chinese.
The physical barriers are considerable however. Napoleon said of the three barriers, mountains, rivers, and deserts that make natural national borders, mountains make minor obstacles while deserts are the most problematic. The Ural mountains was Russia's barrier to overcome, while the Gobi desert was in China's way to Siberia.
Historically the Ming was not in the least bit interested in Siberia. It's the Manchus that were expansionist and prized fur. However by the time the Manchus were in a position to expand north the Russians were already firmly entrenched and it wasn't worth the effort to dislodge them just for the fur trade. The Treaty of Nurchinsk was the result of Manchu-Russian border conflicts and resulted in mutual accomodation.
Not really, the mixed forests of the Amur basin are very different to West or Central Siberia and work better with different agricultural patterns. Plus the Manchus never even got round to using the Amur basin significantly.
Also regarding the Gobi desert and the rivers? What the heck? You do realise the Gobit desert is bounded by the Altai mountains in the north - a massively more formidable barrier than the Urals, and that the Navigable portions of the great rivers are (suprisingly) near their mouths rather than the raging currents they are in the mountains, making it much easier to spread out from the north to the south and east.
Also, the "thin habitable strip" may appear that way if you look at a population map, but there is a "chicken and egg" situation there. The population is thicker along the Trans-Siberian rail line because the Trans-Siberian rail line is there. If it weren't, I'm not sure many Russian colonists would have preferred to move there.
Distance isn't necessarily a barrier to political control, nor is proximity a benefit - as you've pointed out, it's much more practicable to control Siberia from Moscow than it is from Beijing. The Golden Horde is a pretty impressive demonstration, if a bit of a backwards one.
One mountain range is way more of a barrier than several thousand miles of steppe.
I missed this earlier - sure one mountain range is more of a barrier than steppe! This is why I think its unlikely that the Chinese will reach across two - both larger than the Urals!
Also again the thin habitable strip is very much geography based - the forest-steppe is a much better environment for farming and construction, and you saw the majority of russian settlement there long before railroads were invented even when the fur trapping grounds were further north.