Except, the problem is, you need mass to make a kill. They don’t have to be the same size, but they have to be real fucking big - Holdo’s ship, according to Wookiepedia, was about 3.5k long, 700 meters wide, and 450 deep, and it still only managed to break Snoke’s ship in half, not completely destroy it. As I said, we’re not seeing damage as expected by a relativistic impact, we’re just seeing very large objects hitting each other very quick. Too small, you don’t make a kill. So, capital ship, or capital ship equivalent mass of materiel with a hyperdrive, realspace engines(gotta aim it before the jump, after all), reactor, basic sensors and navigation systems, either way, it’s a lot of work, expense and resources that they either can’t afford, or are better served spending elsewhere. Take your pick.
Nope. Chances were, if you tried to take off in a mass shadow, you’d either blow up, or simply disintegrate in hyperspace trying to make the jump.
Lifting into space would be further resources you’d need to spend. Every time we’ve seen something of that scale being built, it’s built in space - so we can assume the cost of lifting it is going to be big enough that even for ships with a lifespan, it’s better to build them in space, let alone a disposable missile.
That’s the one use I can actually see. Ships are one thing, but the amount of damage you could do to a planet? No installation would be safe, so a big enough facility, or city, or an important enough war would justify the use of such a high-cost tactic.
Fuck knows, really, but I assume so too. But I don’t imagine they’re that common as to provide much of an impediment - In canon, we’ve only seen them four times - Deathstars, plus Scarif and Starkiller base.
In legends, we’ve seen them maybe a dozen times, fifteen at most - so I don’t think they’re that common, or at least common enough to be an issue.
That said, it also might not be an issue - remember, TFA, Han jumps the falcon basically right through the Starkiller base shield. It was explicitly made out to be a nearly impossible maneuver, because you’d mash into the planet and die. But don’t need to worry about dying or crashing, if there’s nobody on board to die, and the whole point is to crash.