Rule IV sec 28a. says when only one letter or any combination of unspaced letters is in direct contact with only its opening or closing grouping sign, the ELI is used or not used as though the grouping signs were not present.
P(A and B) the P is adjacent to the grouping sign which is followed by a letter. If you take out the grouping sign the P is in contact with the A -- so that A is no longer a single letter requiring a letter indicator. The B does need the ELI because if the grouping sign is removed, it is a single letter standing alone.
Rule IV sec 25a(vi) defines a single letter (in part) as followed by a space or a punctuation mark in print. A grouping sign is not a mark of punctuation. The P does not need the ELI in any of your examples because it does not meet the criteria for a single letter. Because it is not a single letter, it does not need the ELI.
Does all of that help at all?