I have an alternate theory:
Specht always wanted to use Vic for her career advancement. Marriage was never on the table for her; she just wanted to use her connection to Vic to get her foot in the door and advance her career, then drop him and move on to greener passengers. This didn't pan out, because Sabat was always actively trying to crush Vic's career, so instead of picking him up, getting her career started, and then dropping him quickly she ended up stuck in place right up to the wedding. She didn't ever want to marry him, though, because it's easier to break up when you're not yet married, so when time ran out and 'engagement' became 'wedding', she came up with some excuse to delay it indefinitely.
IMO the reason why Specht didn't break up with Vic back in 2013 is because STC started in 2013. She was already done with him then because she'd failed to get anywhere in voice acting using him, but getting a role in STC meant getting an acting credit (even though no one expected it to be successful in any material way back when they started, it's still an acting credit alongside some other minor actors). She then conveniently broke up with him some time mid-2018, which is around 6 months or so after the final STC episode aired, and she would have been able to determine she'd milked Vic's connections for as much as he was worth (and thanks to Axanar, there was no chance of another STC-esque product out of him)
I believe literally nothing that she says in her emails. She didn't 'discover' any behavior of Vic's in 2019, I don't think she even discovered anything back in 2018 when they broke up. She saw an opportunity to get back into the limelight, make herself look good, and improve her connections in the industry by throwing in with Monica and making up a bunch of nonsense very clearly cobbled together from rumors and nonsense internet stories. I frankly doubt Specht even cares that Vic was cheating on her, because she was probably cheating on him, too. She's a classical gold digger.