In any case, a catastrophic multi-sat collision would only result in a meteor shower
Previous collisions have resulted in debris that intersected with higher orbits. While those debris themselves will decay, if they collide with something in a higher orbit, a significant portion of the resulting debris will be there for a very long time.
Look at the apogees resulting from a major collision in 2009 in fig 3 on page 2
Not in a solid form. There may be some undesirable effects though at greater numbers, we don’t really have good data. Here’s a blog post by the European Space Agency talking about a couple studies on the effects of satellite reentry. Note that the satellites they simulated were significantly larger than the Starlink satellites.