[2020] With very small and easy changes, Conversion trap could become a viable skill!

1) That's not how tags work. Tags don't cause any behaviour, they signal behaviours the skill has. Actually making converted monsters be minions doesn't work under the current minion system and would require substantial changes to minion systems as a whole before we could start on that. This is nowhere close to "easy", nor a "small" change.

2) Not only is that not how tags work, the behaviour you suggest isn't even the behaviour that would be indicated by the trap tag. Conversion Trap is a trap, and thus already has the trap tag. There is no way to just arbitrarily change modifiers that affect traps to also apply to other effects of the skill being used by the trap - specific types of modifiers could potentially be manually applied, similar to how, say, Caustic Arrow has a stat that makes projectile damage modifiers apply to it's damage over time. Even then, this would only apply to the conversion effect, not the monster itself, because that monster fundamentally does not have your trap modifiers, so cannot apply them even if relevant stats are made, so this is only really possible for making trap duration modifiers apply to the duration of conversion. Outside of that, this is impossible.

3 a&b) These changes at least would be relatively straightforward. How "small" a change these represent is debatable at best.

4) Bosses absolutely cannot be freely duplicated at will like that. The only posssible way to do this would be to create an alternate version for a whole bunch of bosses just for the purposes of this skill, with some of their mechanics that tie them to their function in the game stripped out. Again, neither small nor easy.

Not all, and many of those caused weird problems or crashes as well. Map Bosses in general are safer (because of being built in general to support the twinned mod), although there's still a distinction between spawning multiplies at level generation and spawning something during the gameplay which isn't expcted to be spawned that way. But lots of the non-map bosses are tied to their area in odd ways - as one example, any Brutus dying will permanently change the environment/lightning settings for that instance (I know from experience this causes weirdness if you spawn and kill him in the wrong area, but just spawning the extra means that can occur without killing the "real" one). They also grant quest flags, do special drops, and other, weirder mechanics, all of which are tied to being that monster.