1) Lower level androids level up with fewer skill points from your servers, so if you're looking to max out an android's skills the best way that I've found is to build a T1 unit, rank up its skills to 19 across the board and then do a mind transfer into a blank T4 shell.

2) Catastrophic failures during upgrading can kill an active android very easily, especially if it's anything that affects the brain, and most likely if upgrading neural pathways or installing positronic wiring. However death will not occur if the robot isn't active, i.e. a shell unit waiting for a personality transfer. Yes, you can still fail and yes the robot will take surgery damage, but until you give it a personality the damage will not be instantly fatal.

3) In conclusion, the best path that I've found for creating UberBots is:

Level up your servers (optional)

Build a T1 android. Remove its personality now while it is still cheap and expendable. A death here won't be a big deal. If it survives, move to the next step. If not, repeat this step.

While the servers are leveling up its skills, construct a L4 android shell.

Install every upgrade possible into the L4 shell while it is still technically mindless.

When your T1 is at 19 everything and your T4 shell has all the upgrades, then do a mind transfer.

You now have a L4 with 19 in everything, with a +150 mood bonus and every possible upgrade with no risk of death to your expensive T4 unit.

If you have the skill trainer mod (or any skill trainer mod) you can also assign your T4 to it hit that final 20 point skill mark in short order, since it will have all kinds of learning upgrades already running.