in my opinion, the only thing to make not alts-self squad is make the boss more harder and tricky (ex: explode in the circle, aoe atk which need to avoid them), but sure this is not good for low or mid lvl.
So, i suggest to make 2 optional type on dungeon at the begin, 1st for the normal boss , and 2nd one for harder boss. and the reward we talking before only can get from 2nd intance, which is i think its fair cause we cant do alt-self squad with harder bosses and really need team work for that)
although, this opinion still have weakness on it:
1. high-lvl still can solo/alts-self on low cave (maybe from 29-51 caves)
2. low-lvl and mid-lvl hard to find ppl who want come with them
but, all of this are depends of the player itself. cause none suggest have all positive options and everything we do for low and mid dungeons, high-lvl still have advantage on it.
that all i can think, maybe its impossible to make this suggest happens, but for me open share dungeons is a bad idea.