Calculating maximum loss on options spread including early assignment and margin calls

I'm going to assume that you are referring to a vertical spread because some types of spreads such as calendars and diagonals will have variable outcomes based on other factors.

A vertical spread is a fixed position that involves a long leg and a short leg. It has a maximum risk and a maximum reward. Its P&L should be evaluated without other variables such as margin interest (if assigned) because then, it's no longer a spread, becoming long (or short) equity that is put (or call) protected.

If assigned early on the short leg, ignoring fees, the expiration P&L doesn't change because you still own the long leg. There is no simple one size fits all formula for the new position because you may or may not have the margin to support the position and the remaining long leg may or may not have salvage value (time premium).

This might be a lot clearer to you if you set up a position and then examined the various scenarios that you have suggested, and then determine the P&L of the new position(s) after assignment, along with margin charges, etc.