Sadly, Rodney's the one who makes the most sense. But I agree, they'd only use him to show Rush's "superiority" by showing how not even Rodney can solve their problems, and to force down our throats how Rush really is the "foremost expert on all things Ancient" ( yeah, right, I think Sam, Rodney, Radek and Daniel beg to differ on this! ). And they're going to character-rape/castrate Rodney by doing this.
Sadly I don't think David feels about the whole situation as strongly as for example Joe does. Yes, he's been disappointed and seeming like he felt thrown aside, but I haven't seen any direct resentment from him, not to the point of anger. Maybe he'll feel that if he gets to guest-star, that they still appreciate him? I wish somebody close to him ( I don't think he'd listen to an obviously biased fan on this subject, and he shouldn't either, really. He shouldn't be caught up in our conflicts or intrigues. I just wish he knew the truth. ) would point out what TPTB have said about SGA and its actors, the ratings etc. Also, the revelation that an SGU movie would take precedence over an SGA one. Sure, he's a geek and seems to have a bit of knowledge of what goes on on the internet when it comes to Stargate, but I don't know if he reads Mallozzi's blog or the blogs and interviews by current creators/actors/mothers. Probably not. He seems to be still in contact with Joe, I wonder if they've discussed this?
I really think that whoever from SGA agrees to guest-star on SGU is only... well, I don't know if the word "betray" is the right word, perhaps too strong... but I feel that s/he is not only "licking the hand that's been beating them" but also ultimately putting themselves and their fellow actors in a weakened position for the future. It may seem like a good thing for the moment, getting a job and money and some more screen time, but it will help reinforce the trend with at least the current PTB of Stargate ( I don't know about the rest of the entertainment industry ) that actors are disposeable ( until you need them again, and then they should be grateful to be given another chance ), only tools you use to have somebody speak your gritty, edgy, realistic and mature lines, only puppets really and not artists ( unless of course you're an SGU actor, then you're a genius. Well, until TPTB grow bored of SGU. ). Didn't Joe say as much in an interview once: that these PTB view actors as disposeable?
ITA with everything you said. 
and if a looker like Michael Shanks couldn't raise sgpoo's ratings...whiney mckay won't!