Awesome brokerage on Monarchy to get out of that hole right quick. And a shrewd move on researching Banking as well, even if it didn't pay off this time. The AIs just about always prioritize other techs ahead of Banking and it's often possible to get first.
When to self-research can be a tricky decision. Generally, you won't be able to research something first yourself if you're still making significant tech payouts to any other civ. Research can still be worth it, though; it blunts the eventual purchase cost. Often a good strategy is to keep researching yourself; if you don't get the tech first, buy in as soon as two rivals get it, resell it again for whatever you can get, and try again on another tech.
And, whenever you're researching, it's almost invariably right to do so at 100% as long as you have money in the bank to pay for it. In an emergency (somebody shows sneak-attack and you need to rush a defender), you can usually take out a loan from another AI for that turn, and then you can halt research for money. Instead of keeping a cash cushion, it's better to guarantee that treasury cash will be useful, by turning it into beakers via deficit research. Taking the concept to the logical extreme might illustrate the point. If you could snap your fingers and immediately turn banked cash into beakers, you almost always would - so get as close to doing that as you can by always running deficit research. (I know you know most of this, Gris, I'm just generally commenting.
The location of the FP does
matter prior to Communism; it does give a distance corruption reduction to cities in its area. The effect is nowhere near as large as in PTW, though; best bet is just to build it as soon as you can in the direction where you have the most cities. First ring is fine; second ring on a larger map if your Palace is at one end of your civ.
Communism was a great call for your kingdom. Just to go over some of the mechanics for it: Communism's OCN is naturally huge to begin with; the SPHQ is actually fairly marginal. The government gets an automatic +100% to OCN, and the FP (which you'll already have) adds +300%. So the OCN is already 5 times the regular number - coming to 85 cities on this small map. The SPHQ then bumps it by another 300%, to 136. This isn't trivial - of course you should always prebuild the SPHQ ASAP - but it just a contributing factor to Communism's power.
In Communism, all your cities are treated as halfway down the line to your true number of cities with regard to the huge OCN. If you had 30 cities when you switched, all your cities would be behaving as if they were 15th-closest (half of 30) to your palace with an OCN of 100 or 160. That works out to about 8% or 5% corruption from city rank wrt optimal cities: the equivalent of mid-first-ring, minimal indeed. "Distance" corruption is Communism's chief problem - it's a flat 25% regardless of distance and FP/SPHQ. But that can be halved with each of a courthouse, police station, or WLTKD, all stackable multiplicatively.
In a near-complete reversal from earlier versions, the number of cities you have is no drag at all on Communism's productivity. With a small number of cities, your unmodified corruption is 25%. With as many as 68 cities on this small map - which would be almost up to domination - that base only goes up to 37.5% in each city. On any map size, you can't GET enough cities to slow Communism's productivity without just winning by domination.
Whenever your goal in a war is civicide (England), it's almost invariably good to sign everyone you can to alliances. Especially if you're just flipping techs to them and not really paying anything of cost to yourself. If you do get in big trouble, which wouldn't happen anyway with a tech lead against an off-continent opponent, you can renege and sign peace.
Glad you were able to fit the game in and enjoyed it.
You also got to do a lot of city capturing, and that indeed is where the specialists are at their most valuable. Not unlike real life, in fact; ask the Army about the civil engineers and policemen that are rebuilding our conquered cities of Baghdad and Basra.