So what's happening with Sandor right now is that he's on a Rep Building Quest.
The moment he landed on the Stepstones, if he requested an audience with us then he'd probably get one. He's just way too politically relevant as a (potentially former) Kingsguard, too personally relevant as the brother of the Mountain, too metaly relevant, and too likely to be a PC to not talk with him.
Instead he's joined the Melee as both a proving ground for himself to get that sweet sweet XP, and if this was a Video Game the further he got through the Melee the more Rep, Respect, and Fame he'd gain with basically all Imperium Factions and any other factions that aren't innately Imperium Hostile. Winning the entire Large Melee would provide a capstone of Rep gain and an achievement of "Baddest Dude on the Block" sorts.
While he could talk to Viserys at any time throughout this, the longer he waits and the more rep he builds the more Viserys will personally respect him in comparison to his role as a McGuffin and Plot Mover.
Up to four levels of Fighter is generally sense making, outside of fluff related things. It gets you a Bonus Feat every level except 3rd, which is when you would get a normal Feat anyway. If we're building up to 12, something like Fighter 4/Chevalier 6/Warlord 2 would probably fit thematically, representing his original raw combatant state with some amount of leadership experience, building into becoming a Leader of Men (and retraining what was probably originally a Fighter 7 or Fighter 6/Chevalier 1 build into a Martial Class that has Social Skills), followed by starting his first steps on the path to Big Dick Martial Fighting.
I don't actually know Chevalier or Warlord class features, so someone else will have to help out with the actual making of the build.