There are plenty of examples throughout media of a character debuting in a non-canon work and subsequently being incorporated into the main canon.
Harley Quinn originated in the Batman animated series, but has since become a major recurring character in all Batman media, including the comics. Senran Kagura's Ayame was originally exclusive to a gacha spinoff, before making cameos in the main games as a shopkeeper, and eventually becoming an official part of the main cast. It's not an uncommon thing.
There are also examples of characters being intended to be canon from the start, but making their first appearance in non-canon material. Pokemon, in particular, loves doing this, debuting new Pokemon from the next generation in the anime before the next gen games come out. One notable example is that Mega Charizard X debuted in Pokemon Origins, before its existence was even revealed to the public. This specific phenomenon is pretty much in-line with how Lisa was introduced, as well.
Then there are times when a character appears in a non-canon capacity in a canon entry before their proper introduction. Uzume is a dlc character in Neptunia Re;Birth 1, 2, and 3, despite the fact her canon debut wasn't until the sequel to 3, VII. The games themselves are canon (well, arguably in the case of 1, but 2 and 3 definitely are), but Uzume's presence within those games absolutely isn't. This also makes a nice parallel with the 5U/LR characters.
Basically, a canon character's first appearance and a character's first canon appearance are two different things.
Yeah, except in the case of Harley Quinn, similar to Broly in Dragon Ball Super, they had to rewrite her character somewhat for her to count as canon (for example, they made her a bit more nuts in the canon comics than in the DCAU, gave her an extensive makeover that made her look more like Cruella DeVil in a Loli outfit, and more importantly made sure to make clear that she did indeed earn her degree in psychology, whereas in the DCAU, especially the tie-in comics, they implied that she essentially cheated in the course via sexual favors).
And the Pokémon anime, as much as I REALLY hate having to use this term, was largely meant to market the games (mostly in later generations where they for some reason don't deviate from the games even when it makes more logical sense to do so, but even the early seasons were essentially meant to promote the games), hence why they generally give teasers to then-upcoming Pokémon (and that especially includes Ho-Oh, which practically confirmed there were indeed going to be sequels to RGB). And I really wouldn't consider Mega Charizard X's debut to even be remotely the same as Lisa's debut, especially considering that there was really little indication there was even going to be a DOA4, unlike with Pokémon Origins which, aside from it being a retelling of the games, also was meant to market upcoming games.
And quite frankly, I fail to see how the 5U/LR characters qualify as non-canon at all, their debuts or otherwise. While I'm still pretty irritated that they failed to give them story DLC, they still debuted in a fairly canon capacity. If they weren't canon, they weren't canon. It's as simple as that. I mean, did Chris Jenner become canon despite Metal Gear Ghost Babel being non-canon?
Either way, regardless of whether Lisa and Niki prove or disprove DOAX's canonicity (and quite frankly, I'm pretty sure they do prove it. No other reason to have them appear, especially regarding the characterizations given there match up. I know if I were Itagaki and I wanted DOAX to be non-canon, I sure as heck would not allow Lisa or Niki to appear at all, specifically to AVOID giving any hint to canonicity. Yes, I guess you could say I'm a bit totalitarian in terms of canonicity.), Kasumi's ending in DOA4 most certainly does confirm DOAX's canonicity (it literally makes no sense otherwise to have her wake up in Gemstone Reef Suite from DOAX2 in her ending, especially when DOAX2 wasn't even officially announced yet), to say little about Zack's ending in DOAX2. And that's not even getting into how, if Jadeinchains is to be believed, DOA6 actually references DOAX at one point. And let's not forget, Dead or Alive Paradise is pretty much the reason we even have DOA games right now, so that if anything is ANOTHER reason to consider them canon.