A major point. Altitude can be transferred into airspeed. It wasn't until the advent of radar and radar guided missiles aboard fighters that the advantages of starting a fight low came into play in earnest. Case in point, one (or both) of the F-14 engagements against the Libyans in the 1980s saw the F-14s going low on purpose, both to mask themselves against the sea from the Libyans' radars, but also giving their own radars a clean, uncluttered background upon which to see the Libyans.
Absolutely. There are numerous accounts of pilots getting shot down over their own territory who were up flying and fighting again within days, sometimes the same day.