Das, what is the situation in the Flemish Congo like? Is it a brutalist state a la the OTL Congo Free State or is it more in line with a "traditional" African colony?
More of the latter.
How large is the European population there?
Fairly small and concentrated in the coastal areas.
Why is the colony so concentrated along the coast, i.e. why weren't there massive expansion efforts into the interior along the lines of Henry Morton Stanley's in OTL?
Such expansion efforts have only become possible - and indeed started - very recently. Plus there was a different focus in the Flemish colonial policies overall.
How did the Flemish Congo come into existence in the first place?
They established some trade outposts, and alternately warred or traded with the locals. In the 19th century, with the beginning of major colonial expansion into Africa's interior, the Flemings finally got the upper hand and annexed Kongo.
Similarly, what is the correspongind situation in Mozambique?
Similar to OTL in some regards, only the Flemings had paid much more attention to the colony than the Portuguese did, and so it is more developed and has a higher European population.
In the Nigerian and West African colonies?
These have been trade outposts until recently, though now the Flemings have expanded inland, establishing small forts and subjugating local tribes, though those, as well as the King of Benin, had been able to retain considerable internal autonomy.
Why is it red instead of a more traditional Russian color?
Because red
is a traditional Russian colour.

Also, it is the background of Tver's coat of arms.
1) How did it come to control northern Russia?
It was a small, but strategically-positioned trade outpost of the
Slovenian capital, Novgorod. When Slovenia fell apart in the late 11th century, Tver began its gradual ascent, and, thanks to its resources, growing population and good position, ultimately managed to reunite the north (by the late 15th century or so, actually).
2) You've mentioned a Wilhelm II-style government, is this as similar as Social Democrat control of a largely powerless "advisory" body? Are there any other political groups?
I believe I was talking more about the foreign policy. Tver is a very conditional parliamentary monarchy, actually quite similar to Britain in some legal regards, only with the monarchs having much more practical power and using what they have this much more actively. That said, there is an active Arbeitist movement that is represented in the parliament along with more traditional liberals and conservatives. However, a definite similarity to Germany is present; there is a powerful and well-developed military-industrial complex/lobby, for instance.
How do average Tverians feel about their government, and how does the government care for them?
Social programs are pretty underdeveloped (despite recent reforms), but the population is mostly loyal; central and northwestern centers have larger oppositionary movements, but government supporters are more active in there as well. The southwest has more dissidents, though those are mostly assorted separatists. Not very powerful, though.
3) How did the Golden Horde survive, what is its domestic situation, and its relations with Tver?
It survived because nobody bothered to kill it; actually it fell apart on many occasions, but has been recently reassembled yet again. It is fairly primitive and not too stable; relations with Tver are ambiguous, verging between cold neutrality and fervent Tverophilia, especially amongst the local nobles and officials who are often bribed by the Tverian industrialists.
4) You've mentioned its in the middle of an economic boom, how industrialized is it? Does the port on the Baltic matter much, trade wise?
It is very industrialised; that is to say, it does have many primitive rural regions, but the main central areas are industrial powerhouses, though their advantage might be lost soon. The Baltic ports are major outlets of trade, as are some ports on the Volga. That said, I guess that the former at least are not that much on the world scale.
5) I'm guessing from its aggressive nature and Wilhelm II thing that it has a capable army, but is it quality or quantity wise?
It is medium-to-large sized, but also pretty good, though not great.
Is the navy, with only one port, neglected like I'd expect?
Four ports, though it still
has been traditionally neglected. More recently, though, the Tverians began to modernise and expand it at an impressive rate, also buying up ships from elsewhere. There be an arms race in the Baltic Sea.