4: Doublecast allows you to cast ALL spells available to the entire Light Warrior party, no other class levels required. This includes White 7, Black 7, all Summons (including dark worlds once you join the parties), and all Magic Blades. It also includes Holy (but not Meteo). This is a Memorist property, not a Doublecast glitch--when accessing ANY spellcasting through !Memorandom, you get all applicable spells. !Red Magic allows access to White 8 and Black 7, for example. Doublecast can access all spell LISTS by default, so in !Memorandom it can access all spells.
I also asked them by direct message to verify that their characters had just been Red Mage -> Memorist except where otherwise noted, and they claimed they had.
So there is currently a contradiction between your reports. May be @JaeromDarkwind got it wrong, maybe you're confused, and I'm currently confused.
I know that in my posts 5 years ago I used to be under a similar impression as you, that you needed 1 level in summoner, but separate reports have indicated that you do gain access to summons after all, including from a person who ran just Red -> Memorist.
My own Memorist 5 years ago was imperfect, in that they learned a few unneeded abilities including Battle Arts, but still had double cast, so they could do a bunch of the double casting from this build, but not with perfect reliability. That's part of why I found JaeromDarkwind's results so interesting - they had apparently made a pure Red Mage --> Memorist and perfected the double casting.
I wonder if the key is the idea of "Red Magic" vs. "Doublecast". I'm under the impression that Summons show up within the Doublecast command but not the Red Magic command. Did you mean to say "Sarah's Red Magic didn't include Summon until she reached Level 1 in the Summoner job", or did you actually mean to say ""Sarah's **Doublecast** didn't include Summon until she reached Level 1 in the Summoner job""? Could this account for the discrepancy?
I do agree with you that a chief drawback of the build [regardless of which way Summon works] is that you don't get to experiment with the character or learn F-abilities in the way that the others do, as I noted in the caveats section.