Full gold does four armor points more than leather. You're right, it doesn't make much of a difference though . It's easier just to find the gold armor.
I really don't see an issue with a very rare diamond. If there was only two then all you could make is a diamond sword. Miq is more powerful then a diamond sword so it would be less op then a kit that I can use every 10 minutes. However, if there was enough to make armor, that would be op.
If there were two diamonds in each map, I do not see it as a problem because the stone sword MIQ is already more overpowered than a regular diamond sword and the chance of getting both of the diamonds is near impossible.
There is no way to limit it to 2 in one map. Making it rare, so far as I know, randomizes per chest, not per map. It doesn't say "There will be exactly 8 iron swords per map" It says "There is a _% chance of an iron sword in this chest" I might be wrong, so, lets ask @kukelekuuk00
There's many in the spaceship's control room and in the back, near the wings. (near where the big ladder to the spaceship is)