Pretty sure mostodern bikes are double skin
Very few tanks if any have been double skin for the last 50 odd years if ever! I certainly can't think of one main stream bike, quite a few had false tanks older Goldwings had side opening tanks, but not double skin.
mine had similar damage, filler will eventually crack / shrink n show through paint no matter how good a job is done.
Not in my experience, vast majority of dent's can be pulled out and filled, if done by skilled people you'd never know it was there. I work on bikes that are 30 odd years old that I know have had repairs done, you can't see the repair. As with all things that get damaged, repair isn't just about can it be done, it's is it worth it, i.e. a replacement of the same colour be that new or second hand could well be cheaper than quality repair.
Going naked, well that's the cheap way out and will change how the bike feels, frames and firings can be repaired.
Hunt around, go with recommendations and do your sums. Obviously if you can't afford the work you've got little choice, but naked not only will it not ride as well it will sell much lower than standard.