We're mostly on the same page, with a few differences which may be due to interpretation, or even semantics.
I don't disagree.
The cities were republican and liberal hotbeds, and that wasn't news then. The regime played in the countryside instead.
We were then a long, long way from urban populations becoming dominating. It did take well up to the 1930s for that to happen.
The Versailles government held power against the radical, urban republican factions because it had the support of the countryside. And the so called Republicans won against monarchists in 1876 and 1877 and limited the bonapartist resurgence because they were able to expand upon rural electorate, a strategy of pragmatism that to cause a rift between Gambetta 'opportunistic' line and the radical line of Clemenceau.
In 1870, the rural constituences were still a solid base for the imperial regime. At that point, as the repression of the June Days in '48 and of the Commune in '71 showed, holding urban constituencies didn't matter to hold onto power. So far, the only venue of power for Republicans had been through uprisings and coups (the overthrow of the empire in september 1870 was de facto a soft coup against the Empress government).
The thing with the Prussian army was not about it being attractive. The Prussian officer corps was very aristocratic and military service was a tradition deeply rooted in Prussian psyche, a feature that ran all the way from Frederick the Great to Second World War. You would never find such level of militarism in France, and even Napoléon I's time didn't give way to such traditions.
Then, I will hardly consider the weak-willed nature of Napoléon III in his later years a good opinion. I'd rather put things back into their context.
He was weak-willed, but under one faction's thumb, I think not. His will was weak, but he still had one. One to make the 1868 law on press freedom and to push Niel reforms, to concede the government to a parliamentarians. The elections of 1869, the Ollivier government, all happened because he wanted it, not because of the Empress. In the process, he had begun to make amends with Prince Napoléon who was instrumental in acting as intermediary with Ollivier and other opposition figures during the negotiations over the formation of the government.
As for Gramont inclusion in the government and other figures belonging to the Empress faction, it was an effort by both Ollivier and Napoléon III to build a parliamentarian consensus. Bonapartist deputies still had a majority, but a very divided one. Republican support was excluded, and Ollivier couldn't rely on the sole support of liberal independents and Bonapartists as there were just not enough for a majority; he had to make overtures to more conservative and orleanist member of the bonapartist caucus to gather his majority in the Palais Bourbon. That's realpolitik from Ollivier and consensus building from the Emperor.
Same thing for Bazaine. As Napoléon III, he had his own dark legend; it's "vae victis". When you get to picture such a character, you have to be very careful about the part of the dark legend forged by opponents and detractors in the decades afterwards. I only consider the feats of his carreer through the lens of coherence to see through what might have been genuine descriptions and politically motivated diabolisation. The thing is that up so far, he had been a relatively competent military commander who lost himself in pursuit of ambition.
As for his free way to England, there is no dark scheme to look for. The capture of Napoléon III and the overthrow of the empire had deprived Bismarck of an interlocutor to negotiate peace at a time it was within reach, since Prussian objectives had been fullfilled so far. The takeover by Republicans and their hardline stance to continue the war had driven Bismarck to seek alternate ways. Among them were projects of restoration of the empire and overthrow of the republican regime. Bazaine surrendering Metz to return save the Empire from republicans and be its hero was another gamble that fitted the Marshal's persona as I see it. That it went nowhere afterwards is another matter, irrelevant as to judging the man.
Napoléon III's death was a consequence of complications from a recent surgery to remove his bladder stones. If he had still been emperor, that might have gone different.
Not in the good sense automatically. It's possible the operation could have taken place earlier, and he would have died earlier. Or the operation could have been better conducted and he wouldn't have died from it. IOTL, there were two successive surgeries on a few days interval with plans for a third, but that proved too exhausting and was fatal to the exiled emperor. If it had been only one surgery, maybe things would have been different. Who knows? But I haven't searched the subject enough to be conclusive, other than in saying that things could have easily gone a way or another, that Napoléon III could have lived longer or died earlier.
As for the Chalons option, I tended to think as well, but the more I read about military matters in that era, and the less convinced I became. I had read from Eugène Rouher's 1949 biography by Robert Schnerb that MacMahon was in favor of a retreat to Paris as he told a visiting Rouher (I'll have to find the book for a precise quote, it's buried somewhere in boxes).
To us, Chalons could make more sense, but that's us. We have hindsight, and we can play armchair generals (I don't mean it negatively), but we can substitute modern judgment to the rationales of the time.
The French army then was decidedly defensive in mindset, and wasn't used to grandiose strategic maneuvers such as those Napoléon I could set up 60 years before, unlike the Prussians. Besides, the defeats in Alsace and Lorraine had humbled the Imperial army and it wasn't seeking actively to engage the German army.
When it did, due to political interference by the Empress, it led to disaster. And even as MacMahon marched his men to Metz's relief, that was half-heartedly.
Retreating to Paris obeyed to a defensive logic. Chalons wasn't as good a defensive position as Paris was. Far from it. Thanks to Thiers, Paris had become the strongest fortified place in western Europe at the time. Its forts had navy guns, which though they were breech loaded, had a far superior range to anything the Prussians could muster and provided a large defensive parameter for Paris. The concentration of railroads and the Petite Ceinture railroad allowed defenders great flexibility and speed in ferrying reinforcements where they were needed, faster than the Prussians did. Plus, to invest Paris, crossing the Seine river was required and such geography meant a break between the two parts of the investment lines.
The problem of the OTL garrison was that it was essentially militia, unfit to confront the Prussians and German troops in open battle. Because of that, they were unable to prevent the investment of Paris and the siege. An army at Chalons wasn't changing this. Worse, the Prussians were actively seeking such a battle in open field against the remnants of the French army, to destroy it, which they could have done due to superior tactics, doctrine and artillery (they didn't risk much from a sortie by the militiamen of Paris, which they could easily outmaneuver), and at best (for the French), the Prussians would have placed themselves between Paris and the army of Chalons.
The move to Paris was the most sensible move in line with the defensive mentality of French military leaders.
With the sieges in Lorraine and Alsace still going on, the Germans simply didn't have the manpower to invest and besiege Paris with the Army of Chalons in its walls, or at least it was thought so. Paris was the safest option of all.