in both cases, the most likely suspect is a block in your world doing something its not supposed to, or a block that shouldnt exist thats locking it up... etc etc etc. normally i'd suggest using mcedit to rectify that problem, but it doesnt work for tekkit worlds as far as i know. it didnt for mine but you can try it.
what you CAN try is running the world in a normal bukkit server, which will cause plenty of errors but i believe will make it either fix itself, delete all the "mod" items in the world, or at the very least make it openable in MCedit, which will allow you to find the problem and delete it.
even if it deletes all your mod items in the world, it will leave your world intact and you can rebuild off of that, to get more material that isnt vanilla you'd have to wander off to an unloaded chunk.
if that doesnt work then i dunno, mabye someone else can help.