5. It would be very grindy and tiring if only the companions in the actual party would receive xp, a terrible idea in my opinion. Imagine it that way: the companions left behind don't idle in the camp, contrary to what they make you believe because you are not the center of the world and they give a shit about you, instead they do their own adventurous stuff during your absence. They are not love-sick puppies waiting for you, to cite Shadowheart, more or less.
As for many things, Dragon Age Origins has a good solution to this. Companions you don't take with you don't gain exp, but if they ever are 2 levels behind the PC they're automatically leveled up to be only 1 level behind. This prevents them from ever becoming useless, but still means that the characters in your party will be stronger.
5. there should be rewards to travelling and completing quests solo, in terms of xp or anything else. you should level up faster and get perks if your just doing it all solo, there should be a benefit to the increased challenge. travelling around with 4 characters is far too easy.
I agree that smaller party sizes should get more exp per participant, though beware as you already claim to find the game too easy with a party of 4. So a party of 1-3 higher-leveled characters might result in a similarly easy game. Personally, I think the exp given formula should be adjusted so that the game is similarly difficult for all party sizes, and then simply rely on game difficulty modes to change the difficulty.
Of course, the combination of the DAO solution and split exp means that players
can cheese the game by going around solo for tons of exp, then exploiting the auto-leveled companions, but whatever. This means they won't have companions around for early-game story/quest triggers, so it's ~fine.