1. Very strongly - ⍴,'A̲' has got to equal 1 no matter what !!
You may think so, but if you want to be consistent on that, you would have to implement a completely new character set and abandon Unicode.
I'll give you an example. What would you want ⍴,'ä' to be?
Right now, that could return either 1 or 2 depending on whether the ä was using the precomposed character (U+00E4) or the combining mark (U+0061, U+0308). Visually, these are identical, and generally you'd expect them to compare equal.
In Unicode, the comparison of equivalent (but with different characters) strings are done by performing a normalisation step prior to comparison. There are 4 different types of normalisation, with different behaviour.
Now, the ä character has a precomposed form in Unicode, and if you couple that with the NFC normalisation form, you'd get the above _expression_ to return 1.
However, the reason for ä working is only because there is a precomposed form available. The combining underline does not have that. So if you want to suggest that the _expression_ applied on an underlined character should return 1, you also have to provide a suggestion as to what ⎕UCS X should return. Remember that ⎕UCS has to satisfy (X=⎕UCS ⎕UCS X).