its not hard to change new router for house A, just to remember your unifi ID and unifi password.
the current situation you are mentioned is pulling a long LAN cable from house A to house C through outdoor?
So now, you need to get another long LAN cable, just the length for house A to house B. Then buy a small 5-port D-Link or TP-Link network switch (for best is gigabit switch) then put it to house B.

Connect the LAN cable from the main router of house A to the network switch of house B, then put one new router in house B and connect to the network switch with LAN cable. The old LAN cable plug in the network switch too, then pull it to the router of house C.

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Second situation, just pull another LAN cable directly from main router of house A to the new router of House B.

Depends on what you want to do and how many users there are. If nothing special, any router will do.

If you want to control bandwidth to the 2 houses, get something that can do that. I’d recommend a Mikrotik.

I have a similar setup in my house (UniFi 100). One cable to the neighbour on my left and a wireless link to the neighbour opposite my house. These 2 neighbours share my unifi connection. There’s another wireless link to the neighbour on my right. This is for load balancing and as a backup wan. They’re on unifi 50. I’m using an RB750Gr3 as my main router.

Are you charging for these?

Cool, let's say 6 users per house. Total 18, no problemo. Yes, RB750Gr3 is router only with no wifi. It's the cheapest Mikrotik router-only router but packs some serious horsepower (by home user standards). Wifi, you can use back any old wifi router. Save some money that way. Personally I prefer to split my router and wifi as I don't see myself upgrading the router until TM goes above 500mbps. Wifi, on the other hand, gets upgraded a bit more often.

The setup for my house is little bit more complicated than usual. Each house has their own VLAN so we don't see each others devices. And one additional VLAN for management so I can connect to their routers for monitoring and config. You can do all that with Mikrotik, UBNT or Cisco, maybe even Asus BRT routers but for sure Mikrotik is the cheapest of the lot. You'll need to invest some time to learn to set it up though.

Yes, that’ll work. Use the RB750Gr3 as the main router. Do all the heavy lifting here like bandwidth limiting if necessary. The WiFi APs can be anything. Use back the ones you have for sure. Of course I recommend Mikrotik for AP also but honestly any brand will do.  hAP AC Lite is quite affordable, around RM250.

Currently I’m testing an Aztech FG7008GR. This thing has a few quirks but its performance is damn good. From the same spot where my hAP AC Lite, the Aztech beats it hands down in terms of signal and speed. It’s 2.4ghz 3x3 MIMO and 5ghz 4x4 MU-MIMO. In other words AC2400. Just not sure about stability. Will need more time to determine that. More info here: https://forum.lowyat.net/topic/4543383

Hard to say because WiFi depends a lot on your house’s layout, materials and neighbour interference.
hAP ac Lite: Low power. Only single chain 5GHz. It barely covers my entire house with 2.4GHz.
hAP ac2: Latest. High power. Dual chain 2.4 & 5GHz but not available here yet. This looks good. Even the CPU, at least on paper, is as good as the hEX.
hAP ac: CPU not as strong as the ac2. High power and triple chain for both 2.4 & 5GHz.

So I think, if you’re looking for an AIO, the hAP ac2 might be the better bet.

In my house, 20x70, double storey, brick walls, my old RB2011 is sufficient to cover the whole house with 2.4GHz. I use the hAP ac Lite mainly for the 5GHz for my room upstairs which is also my office.