Both good comments-- thanks!

It seems there may be some more optimization I can do using IB, so I'm looking through their Eastern Europe options and comparing them to BA and Malev. I seem to recall that BA used to have a lot more flights that way, and to Africa and Central Asia too, but perhaps I'm remembering wrong.

Is that GUA-PTY IB routing bookable? It does not come up in searches for several random dates I plugged in, although the MAD-GUA and MAD-PTY flights appear.

I also see that IB has a flight to Malabo-- there's no chance I could get that in, right? I don't completely understand the return-to-Africa for one flight rule.

Lastly, I might look at going up to HEL and taking the train to St. Petersburg if I can get a Russian visa before leaving on the trip. Unfortunately it looks like one of the St. Petersburg flights is a non-OW codeshare, so I can't use it as a stop between two hubs... unless I'm missing something.