The Edelbrock likely had percolation problems just due to the entirely contained float chamber, it is a common complaint with them.
The spacer issue could go either way, four hole or open. Open will favor mid and top end power as it allows each cylinder port to "see" the whole carb. Will have virtually no effect on idle, so don't worry about that. The 4-hole will favor low to mid-range. It is always hit or miss on what will work, only testing both will tell you what is better as every engine reacts to it's combo of parts differently.
If you really want a spacer to match your manifold it needs to be a two hole, like an open with a divider down the middle so that it matches the intake, but that is splitting hairs pretty small at this point as far as performance goes.
Open tends to find more use on performance engines, likely why your builder suggested using one if you use one at all. Some engines don't like any spacer, I have seen that happen too.
If you think about it in your terms, the four corner idle is already voided, as the two right barrels feed the right side plenum, and vice versa. All the four corner idle does is allow the mixture in the plenum area to be more evenly spread, it is not like one idle screw is feeding these two corner cylinders, and that idle screw is feeding another two cylinders, and so forth. It doesn't work that way. They just allow a more even mix to be available instead of all of it dumped down just the front area of the plenum. Having a four corner idle is more of a plus for open plenum intakes(and big cams), not so much on a dual plane but it still helps.