1. Typically, the common law for indivisible harms holds multiple necessary tortfeasors jointly and severally liable, so you're right on that one. This differs from the LA example, obviously, in that the ptf could try to recover 100% from one of however many joint and several def's there are. The def. that pays will seek contribution from the other defendant's for their share.
2. Careful with your terms for contributory vs. comparative negligence. If a ptf is found to have played a part in her injury in a *contributory* negligence jusisdiction, the ptf is completely barred from any recovery, even if they played a relatively small role in their injury.
If a ptf is found compartively negligent, then there's a couple of different options, depending on what sort of jurisdiction you're in. Some jurisdictions hold that a ptf can recover as long as they were less negligent than the defendant (LESS than 50% responsible). Others hold that the ptf can recover as long as they weren't MORE negligent than the defendant (LESS THAN OR EQUAL TO 50% responsible). The recovery will obviously be reduced by the percent the ptf was responsible. Pure comparative negligence jurisdictions don't care if the ptf was more negligent than the defendant, which is what you have in LA. Pure comparative negligence systems seem to be somewhat rare.