It might be because I'm using the alpha19preview, but your first point isn't an issue for me at all.
The "rings o' fire" spread according to the size of the object, but a 10 mile asteroid doesn't succeed in going all around the earth. I'm sure it's just wildly exaggerated in the movie. Even Ceres with 1000 km diameter only goes half way. You should try Titan
On top of that, the liquefaction of the crust isn't instantaneous either. If it is for you, you're probably using a time step, that's too large.
As for the temperature itself, yes it goes up pretty fast, because it's the average temperature over the entire planet, so a pool of lava is largely going to factor into it.
Concerning your second point:
Yes, an asteroid heating up, when it enters the earth, would be pretty cool (though I wouldn't want any fancy visuals for that, just the usual heating up graphics).
However, it is totally unimportant, since it would only be visible for objects of a radius under 100 km (i. e. small asteroids) and only if you were watching in real time. On top of that, it wouldn't do much other than looking fancy, since it wouldn't change anything after the collision.
On your last point:
I think one of the problems with high end resolution graphics for Earth (for other objects, there are just no such high resolution pictures) would be, that it would be very hard, to do dynamic craters and climate change, that look like satellite images.