first, the boarding thing. they don't have that ability set up yet fully(if at all), and even if they did. it is not possible to set abilities up to be 1 time shots.
Not quite, if you listened to the single-unit retreat video, the icnreased ability to pass information between galactic and tactical levels will allow us to do that, which was the problem before (you'd only have been able to keep it for one battle). It's also pretty easy to have limits on how much you can use an ability.
For boarding, there's plenty of in-universe units we can use for that function however, without inventing a new type of Star Destroyer for it. For the role of "small, fast Star Destroyer" that's basically what the Vigil, Arquitens, etc already are.