Can I gain a random Ally card from the reserve if the deck and discard do not have one?
I can't find anything specifically about this in the rules or in any official rule clarifications. Given that I'd go with a strict reading of the rules and only allow a random ally to be drawn from the deck or (if none in the deck) the discard pile as per the section you quote.

When in doubt, applying the rules literally is usually the correct option. Especially, if it makes things harder for your Investigators.