How do I open this door on the west coast?

There's a possible interaction between your numbers 1 and 2 that you're missing. The blue blocks only subtract from yellow blocks that are in the same space. Let's work with an example 2 box puzzle. If there's one blue block in one box, and a yellow block in the other, the solution is to go around the edge of the puzzle, not to go through the middle, splitting them up. If you cordon off the blue block from the yellow block, then the blue block doesn't do its thing, so it's not a valid puzzle solution.

So, be sure that you design a solution that has all the blue blocks in the same space as yellow blocks.

And yes, this is a super tough puzzle, it took me quite a bit of fiddling to get everything in place.

As you requested in the comments, here's a quick solution. Took me less time this time around, since I vaguely recalled some of the tricks I used in the solution.

Interaction between the rules

The obvious interaction between the rules you mention is that

you must draw a shape where you subtract the blue dots from the yellow ones.

However, in the (uncommon) case where

all the yellow dots within some borders get 'eliminated' by blue ones, the shape can be totally arbitrary, as there are "no more" shapes constraining it.

This means that for this particular puzzle,

you must make a shape for the three-dot yellow piece, but connect the four dot piece in a single arbitrary border with all the blue pieces.