Since the vote is so close, I... uh, I guess I'll talk about my feelings on the vote for the Sol System idea?
This might seem a bit 'gamey' to try doing two major options 'at once', meeting both the Primarchs and meeting Malcador afterward, and I totally get why some people would think that even if I think it makes perfect sense in-universe. But aside from that point, here's why I really like the idea.
Well, like it beyond all the implications and interesting shit that'd be fun to see and branch out from. Because I'll admit, I love to see whatever will happen in this quest.
Pulling back from the vote itself, something to note is that Kesar has big reason to go to Mars and meet with Kelbor-hal right now beyond the other stuff. The whole deal with Vulkan that the Fabricator-General was and is massively critical to everything that will happen from now on.
This is the crux of Kesar's 'fight' against the Emperor of Mankind to try fixing the big problems with him and the Imperium, he's intervened in that deal and has taken over full responsibility of the Maelstrom campaign that was meant to be Vulkan's 'duty' to distract him from his reforms plans. This is Kesar's weapon and Kesar happily got it from Vulkan and Konrad, but Kelbor and the Mechanicum isn't all that pleased with this.
In Daemon Hunter's (paraphrased) words, Kelbor's plan for the Maelstrom through Vulkan's deal went from an amazing critical success to just a 'good' success. Losing out on this isn't the end of the world for Kelbor and Kesar's relations, he'll understand why we did it even if he's annoyed at the situation, but he does really want to discuss this matter with Kesar later so we'd probably social him sooner or later anyway. And, on the plus side, in-addition to everyone else, Kesar doing this plan has caused Kelbor to fully understand what makes Kesar tick and decide on his (fucking insane) courses of action.
The Primarchs (which would be Mortarion, Horus, Perturabo and Magnus), meanwhile wouldn't be firmly against Kesar meeting Malcador instead of them, but even with understanding would wish that Kesar met with them instead for the big plans they're working on which is why this whole situation is happening. Gotta work together, share all they've learnt, to get through the coming years and ensure this shit goes according to plan.
Meeting with Malcador is, well… that's rather obvious in terms of why we'd want this, in-universe and out, so I won't waste time going over that. And I'd be happy if this option won instead of the write-in.
Speaking of the write-in… well, this neatly solves a few issues, admittedly might create one or two more, and firmly establishes to everyone that this is not something to be brushed aside, ignored, easily defied or something to be given a concession with.
Going to Mars before Malcador will send a message to everyone that this is serious. This isn't some Primarchs getting a little antsy over the situation, no minor complaints or squabble or whatever. Kesar is not backing down, neither is anyone else on his side.
As Daemon Hunter said, this act would be viewed as an 'escalation' of the current situation. And in my mind, that's a good thing. Risks of civil war or not, something has to change or things will collapse. And change can't happen quietly. In-fact, civil war and rebellion is already being risked. The last vote had at least two confirmed options that would have resulted in this, if the letter option wasn't picked instead.
If this just gets brushed aside somehow, whether Kesar is involved or not, things will blow up. Probably with Mortarion first. So doing this ensures that won't happen, for better or worse.
And, well, Kelbor will go from annoyance to losing out on his major success to absolute joy that this situation is happening and that Mars was chosen for the 'secret' Primarch meeting to take place. Keep in mind, Kelbor would ensure (despite close proximity to Terra), that the meeting would be private. Well, not private from him, but kept secret from anyone else while.
Kelbor is even likely to help teach the Primarchs here to help them deal with Malcador about political matters and such related skills that, as even the Emperor would angrily admit, Kelbor is supremely good at. Which, uh… Kesar will need all the help he can get in facing Malcador afterward and, in the future, all Primarchs involved in this plan will need to ensure victory at the Tribunal.