When I make the lekvar (with California apricots) it is a very thick paste. Is that how yours turned out? I was thinking it would probably be fine on its own, but if you’re worried and want to add something you could use a little gelatin. I think agar would be too firm when you’re starting with a firm paste.
If you want to go the buttercream route, you’ll have to be careful not to add too much lekvar/puree to the buttercream or it will break the emulsion. In other words, I’m not sure it would work to fold just a little buttercream into the puree. Definitely try it first with a small amount to see what happens.
The apricot lekvar/puree is lovely with the silk meringue buttercream and with mousseline (I also add vanilla for the mousseline).