Choosing Citizenship for Will

Your citizenship is irrelevant.
Wherever you die, everything that you own in Belgium will be dealt with under Belgian law.
But go and ask a notaris. Anything that I - and anyone else who responds - says may be wrong as we speak from our understanding rather than from expert knowledge.

b) yes, therefore a) doesn't matter.

However, the taxation of the estate will be under Belgian law, therefore you really need to talk to a Belgian notary to get a proper will drawn up.

"people can choose whether the law applicable to their succession should be that of the country they have last lived in or that of their nationality"
https://ec.europa.eu/info/policies/justice-and-fundamental-rights/civil-...

Speak to a notary. You pay nothing unless they draw up papers for you and charges are reasonable. Not like lawyer fees.

I had good experiences with this one who have specialists in international scenarios
http://www.berquinnotarissen.be