1> I have found that there are many cases where there is no match to be found with calibre+bullet weight+bullet type+powder combinations. Do you guys find it easy to and safe to match all of the above with the nearest match and just go with it? Like, 100grain round nose in Magtech not listed but another make is listed under same weight...
As long as your starting at the beginning load you shouldn't get into trouble as long as the bullets are generally equal. Jacketed and non jacked bullets are not equal. Close but no SEEgar.
2> Can a too snug of a fit bullet, cause a case to show signs that might otherwise look like a too heavy load? Primer rear expansion/case bulge?
A bullet that is too large can cause some grief like not chambering. Most often when loading bottle neck brass a bullet that seems large when you seat it is a case that was not chamfered correctly.
3> in my 357 mag reloads, a formula is listed for the powder I use. The powder only fills the case about half maybe less. Is that a problem?
It's only a problem if you don't pay attention and double charge. ( put powder in twice thus getting a charge that is twice the charge.) this is bad JU-JU and can lead to Ker BOOMS.