The front and rear mounts are there to hold the torque of the engine while driving. In a manual, they do get beat up a bit faster than the auto. Worn mounts wear out other parts faster due to the excessive engine movement, but the engine isn't just going to fall out if you don't replace it.
The rear mount isn't the hardest thing in the world to replace, but you do need to take off the two lower crossmembers. A few of the bolts will be on there TIGHT, so you'll need a breaker bar or decent impact gun to remove them. The engine will hang safely by the main two mounts, and you won't need an alignment or anything afterwards.
Steps (from memory, may miss something:
- Remove E-W crossmember - 19mm x2 on each side, 1 or 2 14mm in the center
- Remove N-S crossmember - 14mm engine mount bolts (under plastic covers) 2x front, 2x rear; 14mm mount bolts, 2x front, 2x rear; may have a 3rd lower mount under there to remove too (don't think its there on the manuals?)
- Remove engine mount through bolt - 17mm bolt/nut
You'll need to re-assemble many of the parts loosely before tightening things in groups to get everything to align.