I agree about the belly scoop.
Far as I know, the trim tabs could be set at 'Neutral', and adjusted as required by the trim controls on the console, port side of cockpit.
For filling the wood grain, in the days of all balsa flying models, this was done with sanding sealer and / or banana oil, although I don't know if these products are readily available these days.
However, an easy alternative is to mix gloss varnish (Polyurethane or similar) with talcum powder, to create a thick porridge. This is then brushed onto and into the grain and allowed to set hard. Once set, it is sanded smooth, and if required, further coats are applied and sanded, until the surface is smooth and all the grain filled. A final coat of varnish can be applied to provide a glass-like finish if desired.
This will give a good, smooth surface for painting, and a thin coat of primer, before the main colour, will ensure a good, smooth, hard paint finish.