I'd like to forget. It is really embarrassing that anyone at IDW thought that Deviations issue needed to happen.
Yeah, there were a million other ways to do that concept of "What if Optimus survived TFTM?" without needlessly bashing Hot Rod at the same time.
It would have been far more creative for Deviations (or, hey, even for TFTM itself!) to have simply had Optimus retire from Autobot leadership once Hot Rod was revealed to be the true chosen bearer of the Matrix, and become Rodimus's second-in-command and chief advisor.
The Diaclone toy that would become Ultra Magnus could have even instead been sold as an upgraded armored version of Optimus, representing the form he took on after stepping down to become Rodimus's sub-commander (maybe named "Optimus Magnus" since Rodimus would be the new Prime). While this would have removed the new character of Ultra Magnus from the equation, the new toy would have still been sold (preferably in its original, more-Optimus-like Diaclone colors) in stores at the time. It also would have paralleled Megatron getting a new upgraded toy of his own as Galvatron (and kinda Hot Rod upgrading to Rodimus).
Like, start the movie as normal, up until the attack on Autobot City, where instead of Ultra Magnus it is now either Springer or Blaster who is in charge and gives the other Autobots the orders that Magnus gives in the movie proper. Then, when Optimus arrives, after saying his "No matter the cost," line, he would turn to the others exiting the shuttle and ask if his new armor is ready to go. They would give it to him, and he would say, "then let's transform..." (transforming to truck mode, his new trailer would attach to him) "... and roll out!" Cue the Stan Bush song.
Optimus drives through the city, guns blazing from the missile pods on his new trailer, blasting Decepticons left and right as he transforms, leaps into the air, and continues to blast his way to Megatron. The two leaders then have their iconic fight, but with Optimus revealing to Megatron his brand new battle armor, showing Megatron that he means business this time. When Hot Rod interferes, however, he does not grapple with Megatron, but instead trips and winds up landing between the two and inadvertently taking Megatron's gunshot meant for Optimus. Megatron laughs maniacally that this unexpected direct hit of his. Enraged at Megatron shooting Hot Rod in the chest, Optimus pummels, Megatron like he did in the movie, sending the Decepticons into retreat. However, Optimus still took some heavy damage, and he collapses as he tries to reach for the fallen Hot Rod.
Instead of a death scene for Optimus, we get a scene of both him and Hot Rod on repair beds, both being repaired by Perceptor. Seeing a light glow through a blast hole in Optimus's chest, Hot Rod inquires about it. Optimus opens his chest and shows him the Matrix, telling him about the prophecy of one using it to light their darkest hour. Optimus momentarily stirs from the pain of his injuries, and accidentally drops the matrix. Hot Rod reaches for it and catches it, having his glowing moment that foreshadows what is to come later, and gives it back to Optimus, who places it back inside his chest.
When the Decepticons later attacked Autobot City again, Optimus and Hot Rod are back to working order, and Optimus basically takes the role of Ultra Magnus from here on out, going in one shuttle with the others while Hot Rod, Kup, and the Dinobots take the other.
By the time of the battle on Junkion where Magnus tried and failed to open the Matrix, it would be Optimus in his new armor who tries and fails to open it. And just before getting blasted to pieces just like Magnus did, Optimus's final words to Galvatron would be something along the lines of "You may finish me here today, Galvatron, but there is only one who can use the Matrix!" And also just like Magnus, the Junkions would rebuild him, and give his body a new coat of blue and black paint; this would be how he gets the color scheme of the Diaclone Powered Convoy cab.
The rest of the movie then plays out with Optimus in the role of Ultra Magnus. After Hot Rod reveals himself to the others as Rodimus Prime, Right after Kup says "I knew you had potential, lad," Optimus would say something like "So did I, Kup. So did I." The movie would end the same, with Rodimus giving his speech, and Optimus now standing beside and slightly behind him, showing his full endorsement of the new Autobot leader.
As for Robert Stack, either he would simply be absent from the film, or he could play Springer, with Neil Ross still voicing him in season 3 and 4 (Springer and Magnus's voices always sounded similar to me in the movie).