There are many safeguards you can put into place, individual charging of cells, having similarly classified cells (each cell is going to have slightly different characteristics).
Having over and undervoltage protection will help limit problems with the batteries.
What you are asking for is a very difficult thing to answer, I worked for a LiPo battery company for almost 2 years and most of their research was focused in 2 areas, energy density and safety, there are many formulas that are very safe even if the battery is punctured 100% of the way through, but they will have lower energy density than the ones that arent as safe. (so far anyways)
There was a whole department designated to destroying the batteries in as many ways possible (crush, nail penetration, slice, over heating, overcharging, over discharging, etc)
to answer your questions:
1. You cant be sure of this with any battery, but with monitoring circuitry you can lower the risk, 4.2V is a bit high voltage for standard LiPO, their range is typically 2.6-3.6V there are different formulas that will easily go to 4.2 but those are more the "oxide" cells which are a tad bit more... volatile... so to speak.
2. As stated above this is dependent upon the cell, also state of charge, temperature, etc.
3. Over heats is a relative term, some cells are perfectly happy to go all the way up to 150-200C BUT charging or discharging them at these temperatures can cause internal damage usually leading to lowered capacity and sometimes what are called "vents" which is when the pressure inside is enough to cause the battery to leak its electrolyte out without exploding.
4. Some cells are fine with shorts, not for long though, almost any LiPo cell will recover almost 100% after a momentary short (<250ms), longer than that and you either start melting things or causing internal shorts, either way you will have a diminished capacity.
5. Charging backwards is probably the best thing that can happen to the cell of all the bad things, this will firstly discharge the capacity to 0, then it will vent and cause an open circuit if youre lucky, if not it will start charging capacity backwards which, depending on supplied current, may either cause a vent, a meltdown or something a little more violent.
6. With properly balanced cells, and voltage supply, this should be no problem at all, theoretically they should self equalize BUT they are all so different you sometimes get cases like you were saying where one will charge and the others will not.
Please note that in most cases the electrolyte used in LiPo batteries is HIGHLY flammible, so be conscious of that.
