Can anyone explain to me how to earn money in Nexerelin?
Same as vanilla: bounties and trade.
Which actually is a problem as far as I'm concerned - Nexelerin provides player with stuff to do late game, but it's not financially rewarding. Early and mid game is exactly same bounty hunting as always(I'm not much of a trader, and that's the only real way to significantly profit from combat).
Having player salary increase with faction size and providing premium on conquering a planet(max if alone, less for invasion fleet support) would be nice. Maybe also refund (partially?) successful invasion fleets sponsored by player.
Anyway these are details, point is - there is not enough incentive for participating in faction warfare, especially early. You don't even need to defend your faction - I've never seen size 7 worlds conquered (unless I'm doing the conquering). And as long as HQ is safe (main shopping place), player doesn't particularly care for losing size 3 worlds.