This is a huge "what if" and is posted more for fun than for anything I believe to be the case, but as Tom would say, "Consider this:"

Forever Tom always knew that he would never rise above the rank of consigliere, and he als knew that the only person in the family who really loved and protected him was Vito. He also had to know that with Vito gone and Sonny running the show, the family would go into decline.

The last truly gallant act we see Tom perform is figuring out what to do with Woltz. This is while Vito is still alive.

After Vito is shot and presumed dead, who does Sollozzo contact?
Tom, who he sees as the weak link. It doesn't take a lot of convincing to get Tom on board with the idea of selling SOnny on making the deal on drugs while they think Vito is dead. When it turns out he is alive, Tom is still pushing hard for Sonny to make the deal, telling him "If the old man dies, you make the deal."

As consigliere, he allows Sonny to order Tessio's men to act as guards at the hospital when he knows full well he has the power to use armed private detectives to do the same thing without any possible interference from the hospital, yet he doesn't do this until Michael figures out that Vito has been left unguarded.
After Michael is slapped around, Sonny goes after Bruno Tattalia, and at that point, when Sonny is actually getting the upper hand, it is Tom who is still pushing Sonny to go for Sollozzo's "truce." It takes Michael to figure out that Sollozzo doesnt want any truce and that the key for him is Vito's death. Tom scoffs at this idea and has to be talked into it by Michael.

When Vito is brought home, and the war is sill raging, Barzini who hated having his picture taken gets his mug on a NY Taboid, and the crackdown continues. Still by his own admission, once the word gets out that McCluskey was a dirty Cop Tom keeps pushing to have Sonny make some kind of truce. When Sonny says he wants this ended with Tattaglia's death... something that might well bring Barzini to the table, Tom rejects it out of hand. Sonny rightly points out that if the Corleones are not making money no one else is either, and from what we see the Corleones are not losing their muscle.

Tom also had to know that Sonny's temper could get him in trouble, and he also had to know that Carlo (who he suggests to Vito that he be given "something important") is a serial wife beater.
Look at his half ass attempt to stop Sonny from leaving the compound the day he is assassinated.

Only after Michael moves Tom out is a plan hatched to win the war.

In Gf II, Tom is at it again. He drops the ball with Geary; he gives Michael god-awful advice about making an incriminating statement when he could have taken the fifth...which is especialy bad advice when he doesnt know whether Pentangeli is dead or alive notwithstanding that he was able to pick up the ohone and easily find out from his people in NY when it suited him; after Michael is exposed to 5 counts of perjury he tells Michael Fredo doesnt know anything (when in fact Fredo knows Queastadt is on Roth's payroll) he is having an affair, presuaby with SOnny's widow; and even when everything is still closing in on Michael he tries to talk him out of killig Roth.
Yes, he does do a great job to getting Pentangeli out of the way, but that only comes after Michael almost tells him to get out of the organization.

His motive? Money. With Sonny out of the way, and believing Michael would not go into the family business, he could have handled Freo and Connie and taken over financially from Vito, and basically given up all the family muscle to Barzini. Once Micheal enters the picture, there is a constant truggle between them, and neither fully trusts the other. If he can move Michael out in II, again he can control the purse strings. Hell, maybe he even knew Fredo was talking to Ola and Roth ahead of time. I also thought he was a bit too cozy with Kay. As a lawyer he may have helped her get what was then an illegal abortion.

So after all is said and done could Tom have been a mole? There is evidence to suggest it, and if that's the case, and he turns up dead in GFIII do we have reason to say "That was no heart attack?"

A lot of thought on Tom's character, also by me.
At first we should all consider that Don trusted him.
His love towards the family is undoubtful, in my oppinion.A true and innocent love, which sometimes made him weak.
Not a wartime consigliere, as Michael rightly states.Not a Sicilian consigliere, as Santino rightly states.
But never a traitor."Michael I have always been loyal to you,why do you do this to me?"almost cries at the end of #2.
I think that he was the voice of reason and logic, but in a world where reason and logic isn't always the right thing to do.
Would be more helpful in #3, as a lawyer.
On the other hand,let's take under consideration the real consiglieri:their role was something like Tom:to advise their Don by reason.
Overall, I think Tom was a good guy.Not good to be a Don, weak perhaps, but Fredo was also weak...

Do you mean, Neri got him?

A brilliantly construced hypothesis, dt. But it's inconceivable that Tom was ever disloyal to Michael. He made some bad judgment calls because, as Michael said, he wasn't a wartime consigliere. He was a lawyer, trained in conciliation, and the judgment calls you cite are those of a serial conciliator. I think Vito wanted it that way: he put Tom through law school, and trained him as consigliere, because he saw Tom as being helpful in making the family "legitimate"--his long-term goal. I think he also saw Tom's legal training and conciliatory ways as an offset to hot-headed Sonny's tendency toward violence. Since Tom and Sonny were truly brothers, Tom would be influential with Sonny.

What Vito didn't anticipated was that things would get rough, that Sonny would be killed, and that Michael would be drawn into the family business. And Tom, rewarded by Vito for his conciliatory ways, continued them under Michael, who didn't choose Tom either as a brother or as consigliere. That doesn't excuse Michael's wretched ingratitude toward Tom's unflagging loyalty. But it doesn't change the fact that Michael was right when he said Tom wasn't a wartime consigliere.

You are right it is just a hypothesis, but lets put Tom on the couch for a second. Could he have wanted Michael to fail subconsciously?

I agree that his training as a lawyer was to be a conciliator, but lawyers also have to know when conciliations is not going to work and they have to "go to the mattresses." Tom never seemed to have that side to him. Perhaps on some subconscious level, once Sonny died, and after Michael so brutally told him in fromt of everyone EVEN CARLO..."You're out" that at some level he may have wanted Michael to fail.

If it was, I'm sure it was unconscious. Tom's loyalty was based on total dependency on the Corleones. Sonny had rescued him from the streets--even possible starvation. Vito virtually adopted him, put him through college, made him rich and important. But it all hinged on being inside the Corleone family, where he'd spent his entire life--and with which he was totally identified (as in the FBI chart). I don't think he could have functioned on the outside.

Point taken TB. Maybe Michael gave him Stockholm syndrome.

Tom was Irish. Now, how effective could he be at anything let alone at being a mole. Could he even spell it? Almost all his life he had Italians taking care of him. If he hadn't, where would he be? Maybe he would have become a river dancer or the Notre Dame mascot. But, regardless, he could never not be Irish. That's quite an obstacle to overcome. Maybe he could have become the Mr. PotatoHead champion. Who knows? But a mole? Madonne!

He was German-Irish.

(In my best drunken Irish impersonation)

You sir, are a lying member of a no-good race of thievin' Wops!