The only reason these problems exist is because the valour system has turned into utter bullshit. It used to be losing team only and those who got valour would maintain their multi instead of losing it. No idea why valour is now awarded to the winning team and +1's the multi, it's dumb and unnecessary. Even worse, the losing team star players often do not get valour because they get outscored by the winning team, entirely defeating the purpose of the system. The purpose being, rewarding those players that did a good job but lost because of a bad team.
Example for siege: Your team just barely gets a foothold in the defenders castle, you are the only one who gets anywhere. Your 80 score however, which is easily twice the average of your team, is eclipsed by the 100+ scores of multiple attackers who camped a ladder the entire round. Same can happen on battle, where one team's melee rapetrain completely obliterates your team and you got 25 points which is easily twice the average of your team, but they all got 40-50 points with proximity from the rapetrain.
Valour calculation does not look only at your team, it looks at the averages of both teams and rewards the players that have twice that average, which is completely dumb when teams are very skewed. The team that loses gets fucked over twice, because apart from getting wrecked, their star players don't get valour because uneven point distribution between the teams. Therefore only the losing team should get a valour calculation so the valour actually does what it is supposed to do. Save the multi of the people that deserved it.
Your suggestion is terrible even though I would benefit extremely from it. It encourages valour whoring like crazy and it is already bad enough. Doing away with the entire multi system would be the best course anyway.