... unless your PA is a lawyer.
In a nutshell, legal advice has the following characteristics:
Requires legal knowledge, skill, education and judgment
Applies specific law to a particular set of circumstances
Affects someone's legal rights or responsibilities
Creates rights and responsibilities in the advice-giver
Unlike legal information - such as information posted on a street sign - legal advice proposes a specific course of action a client should take. For instance, it's the difference between telling someone what to do (legal advice) as opposed to how to do it (legal information).
For your specific questions
Can they research the CC&Rs, Bylaws or other such states, federal statues to help me better understand the letter?
Researching and even summarising is not legal advice. If they give opinions on what you should do as a result of that research it is legal advice.
Can they write up a response letter that I would look at and approve ...
... or if minor enough they could just respond?
It depends on what's involved. If it is a purely factual response ("Is this your car?") then this is ok. If it is legal advocacy, it isn't.
Putting aside the legalities; how does your PA feel about you suing them if they stuff any of this up? If I was your PA, I wouldn't be acting as your agent without an indemnity.