Not sure about the illegitimate son bit...
At the end of his codex entry, it says, "There are rumors that (Aricellus) has at least one other son through a mistress." If we assume that statement is a Chekhov's gun as opposed to red herring, Felix is the only viable candidate. Unfortunately, the codex is a contradictory mess, and the author is prone to retconning; therefore, it is possible that sentence means nothing.
All he needs now is manuever MC to give Felix orders to March the army East, so he takes command, but does not march, instead he storms the palace. Obviously the question arises on how he'll persuade the soldiers who admire and respect MC to turn against him?
Looking at the historical examples that inspire this game, a soldier's loyalty is mostly based on who provides him with the most loot. It is possible Felix is conspiring with Avila. If given control of an army, he could distribute a bribe among the soldiers. Another option, which doesn't preclude the first, is Felix marches the army into the east, and instead of fighting Avila, he sacks some relatively unguarded wealthy cities under the pretense of suppressing a rebellion. This would undercut support for the protagonist in the east, and after letting his troops loot said cities, Felix could turn around and march on Caudium.
My point being, all of MC's and Marius' fucking is taking place in the palace, and if the palace doesn't know what's going on, how can rank and file soldiers have any knowledge beyond some scant unverifiable rumors that MC and Marius truthfully like women?
We've been told that Cassia is loud enough to be heard outside her chambers, and the protagonist fucked her while there were guards literally stationed right outside her door. There are at least some soldiers who have reason to suspect that the MC likes women, and they've probably spread stories among others.
the strongest support in the empire that confessed in front of the MC to be plotting against the Kings Family
Ironically, the Aristhenes did the exact same thing that the MC did. The protagonist was plotting with Aelinia and Lydia to prevent Marius from attaining the throne, and ended up setting himself up as the senior partner, with all the real power, in charge of the government—just like Aristhenes.
you know messuring his motives against modern world standards is foolish.
Nicabar got sick when Felix arrived back, could be his fault could be a coincident, but the doctors said its fine and he just needs rest, they should be able to spot if someone got poisoned they are palace doctors after all. Also to note no one else got sick, so if he didn't poisoned Nicabars personal stash of whatever its highly unlikely that no one else shows symptoms.
Assigning modern competency to medieval doctors is also foolish: these are the same types of doctors that would ascribe illnesses to an imbalance of humors and evil spirits. Given that diseases spread faster than poisons, it is even more suspicious that a large number of troops are also not afflicted by whatever Nicabar has. On the other hand, this is a world with literal gods. It could be a case of divine interference.
He wouldn't. Not only are this the MCs Legions, there are also the Palace and City Guards stationed and Nicabars troops. So pretty much every non-essential combat formation of Caudeum (we know of) is right now stationed at the Capital. Its the absolutely worst time in the history of ever, you could pick to make a coupe against the MC specifically.
Even if MCs legion gets brain rot and decides to Kill him for no reason but that Felix told them they would be out numbered severely.
Most of the troops stationed in the capital are there in preparation for the campaign in the east. Whoever is given command of those troops will have the bulk Caudium's mobile military. The protagonist gained their loyalty with a few victories over Avila, conquering some land in the Caucasus, and selling some slaves; selling slaves made Caesar the richest man in Rome, but Gaul was much larger than Alania, and even Caesar had legions defect to Pompey. If Felix leads the army east and is "more successful" than the MC, he will have the most powerful military force in Caudium loyal to him.
MC can simply say he had no idea about Marius deviancies and he has soldiers (that know about his involvement with at least Valentina), the Princesses themselves and the Queen to back him up that he is straight, even without him sleeping with all women in an 10 mile radius that plan is more than just weak its outright moronic.
The MC having the royal family, on whom he is basing some of his legitimacy, all come out admitting to being his whores might not be the look the MC wants.