If I understand you right, those necros were not part of the quest. They were simply roaming around while you were heading to the location of the quest (otherwise they would not follow you on the world map).
You could as well run in to them while doing anything else in the game.
As for not able to evade …you can always hit retreat button once in the battle. Yes, you would get your mercs injured repeatedly since they would catch you again, but those injuries does not stack, it takes the same time to heal single injury as it take to heal five injuries (approximately).
So only issue would be loose of the morale and it would be race to find some allied party before your mercs start deserting you.
It’s not automatic game over. Even if you would loose most or all the mercs it would not be automatic game over unless you have no spare equipment and no money at all. You can almost always recover.
Gambling? Yes, luck plays role in this game, but more often it’s your own decisions that matter. You seems to insist in your post that game should be somehow fair and balanced. I disagree. It’s an open world sandbox game, fact that you can run in to enemy which is well beyond your ability is something I consider good, not bad design. Fact that enemies can overpower you IS fair. Because you can also overpower them. Would you complain on the fact that you as a player took on group of 4 enemy tugs in rags and clubs with your party of 12 veterans? I don’t think so.