One problem is too much turnover. The new PC's are not invested in the group. Even if it is the same player, the character has no investment. Personally, I would have either wrapped it up when Leska was taken care of, or I would have already been putting a new story in place before that occurred.
So, how do you rectify the problem? Well, you just want to play and have some fun right? So, play something fun.
Convert something like Tomb of Horrors, or White Plume Mountain, or something equally extravagent lethal. But, don't do it as a mega-adventure. Give the players the setup and allow them to choose to use their existing characters, or to create a new high-level PC to deal with the problem and retire the existing PC. If a player retires the long-term PC, wrap up a few things on what the PC will be doing in retirement. Then, cut them loose in a lethal dungeon environment.
Alternatively, if the group is willing to retire the PC's and start again, then start a new campaign. But, start it off at level 3-5 so they aren't too fragile.
Finally, maybe somebody else is willing to run a game and you can join in as a player.
It sounds like you are kind of looking to keep playing through the semester, but you don't want to leave it completely open-ended in case you move after graduation. So, a long-term, involved storyline is not a good option.