I'm installing HardiePlank siding to a sunroom. It has lots of windows and some are very close. I'm using the method of putting the white trim over the Hardie Boards not beside them. (This is the way a contractor previously did our garage you can see in the background.) I will be putting trim around all the windows. These close together windows will have just this trim showing between them.
Yes, but I’d use an exterior plywood product made for that type of application, like Medium Density Overlay (MDO) plywood and I’d be sure to pre-prime the edges. That is to say, I’d paint primer on the edges before installation.
In order to seal the window edges and building paper, I’d use a peel and stick seal tape.
Jack is correct that trim is customary for spaces up to about 11”.
If you mean narrow spaces up to 8", maybe even up to 11" wide? Yes trim can be the only material there. It is typically done that way anyway.