Levothyroxine will not likely help you lose weight or help water retention. It's just T4, the non active thyroid hormone. The body must convert it to T3, the active hormone.
T3 (cytomel) has a half life that is very short, about 4.5 hours. But it is the active hormone, so weight loss likely requires T3 done properly. I don't convert T4 to T3 well at all. I've been considering going on a T3 only protocol.
Yes, the vials of T will last.
By the way, I retain plenty of water if I inject more than 80mg per week.