So, I'm sure this has been asked before, and I did look but I couldn't find it.

How does split and merge work with multiple point line figures? This came up with the new avengers. I want to split the 300 point Avengers Prime into some of the new figures. The click number on the Prime figure is 4, so if they split at 300 points full health, are the new guys brought in on click four?

Similarly if I have 300 points worth of avengers, and Cap is on click one, how do they merg into the 300 point avengers prime since click 1 is used for the 400 point dial?

Bolded sentence should explain what you're asking. Merge/Split does not compare printed click number on the dial, it instead compares number of clicks from the starting line.

Way to ignore the part earlier in the ability that mentions point cost.

That certainly does explain it! Thank you very much. I suppose when you use it you can pick up hte figure to check how far from the starting line it is

I believe active player decides.

Straight from the description of the Split-ability:

Replacement character(s) are assigned an action token and can’t be
given an action this turn.

They each get one

They each get one.