Foreign vs. domestic issue aside, it seems odd that paying someone money to make a candidate look bad (falsely) could be okay, but paying someone money to keep a candidate from looking bad is not. Two sides of the same coin it would seem to me. If Hoffman was Trump's buddy and had done this to a Democrat candidate, I'm betting he would have been in Mueller's crosshairs by now, and I doubt that a simple apology would've been enough to let him off the hook. I guess we'll have to see what comes of it, if anything.
If I, unaffiliated with any campaign, talk about how bad Trump is during his election campaign, that's not a problem. Heck - many INGOers helped Trump's campaign. Nothing wrong with that - even if it involves blatant lies or coverups.
One of the keys to an investigation like that - from the Federal Election Commission perspective - is whether there was coordination with the campaign. If there was, and it wasn't reported as an in-kind campaign contribution, then laws were probably violated.
Presumably, the campaigning INGOers didn't have any coordination with the Trump campaign, so there's no problem.
Same for me. If I say all sort of bad things about Trump, even if they are false, without coordination from another campaign, there's no problem from the FEC perspective. There could be issues of defamation, but honestly - I don't even need to make up bad stuff about Trump because there's plenty of true bad stuff.
So for bazillionaire dude, if he wants to spend his money spreading manure about Roy Moore, he can kinda do that as long as it isn't connected to the campaign of Moore's opponent(s). Now, there's a ton of FEC rules about what organizations can do, and I'm not nearly familiar enough with all those to know whether bazillionaire dude ran afoul of them. But, an investigation should be able to figure that out.
I'm just describing the general parameters of how it works.