My suggestion for C4/mine problem is implementing these things:
-Unbind planted remote_C4s from overall mine counter;
-Set 6 planted remote_C4s per player personal limit;
-Teammates (may be engineer classes only if you wish) can see who's planted particular proximity mine when looking at it (if there's lots of useless mines, they would ask creator to stop spamming mines or votekick if he continues);
-Techs/Hotwires can disarm friendly proximity mine with secondary fire if they don't like its location.