Once again, being the bearer of bad news but there is a toggle to join war or not. There are many alliances who thrive on not being in war, my friends over in 1134 are a good example, they don’t play war and only do top 5 bounty, they have never been happier. People who only want to play specific events clump together into their own alliances, trying to split your alliance up like that to those who want to play and those who don’t want to play is simply a strange concept. If they want to play then they can use their BP, if they don’t want to play then they won’t use their BP. However your last point there I suspect is what the post is really about, you don’t want to kick your players who are not playing in war but you are struggling to compete against other alliances who have all their members playing. I will repeat my last point that if you want to compete in ALLIANCE WARS then you have to build an alliance of players who want to compete.
The potential for abuse of having a 50m alliance full of level 100 2m+ players choosing to only play with only 15 members and being matched with alliances at 30m who have no level 100s is astronomical. So this idea of yours I am afraid would only amplify the problem you are trying to solve.
Also regarding members who are kicked getting rewards, if a player is kicked then they are kicked for a reason, being kicked is not a pleasant or nice thing, they don’t deserve rewards. If a player voluntarily leaves an alliance and joins another then they can collect their war rewards from the alliance tab at the end of the week, as they normally would.