I'm not sure if you really liked the movie or not now!

But be that as it may, I'll see if I can answer some questions (and this is a way better film than RETURN OF GODZILLA could ever hope to be, simply because of the music and the director, and the good actors and... well, I'll leave it at that).
1. Easily explained- this is a stand alone film, the Kilaak didn't know that Ghidrah was run off by only two monsters in the past- or perhaps it had been more monsters in this "continuity".
2. I thought the gunfights were cool, but actually I wanted lasers when growing up. Too much Star Trek, I guess...
3. I think my ol' pal JFP had TIMED the monster scenes, which I think he said were about 30 minutes of the 89 minute film. That's over a third of the movie, and it's not like the monsters are just in the last 30 minutes- it's nicely spread out.
4. See answer #1- stand alone film. If that's not good enough, who knows how fast Godzilla's grow? Thirty years isn't a long time for such animals at all, I'd bet.
I don't own guns- so you are safe that way!