For the most part, you're interested in the core components because these are the ones that will be damaged by excessive heat:
CPU: The main 'brain' of the computer. This is the part that will usually fluctuate in relationship with the load on your computer. Both sensors are measuring the same thing, but from different places. It's probably a safe bet to average the two.
Northbridge: This is the part of the computer that connects all the other parts together. This chip can get hot, and can (like any component) be damaged by heat. The same applies about the multiple sensors: they're all measuring the same thing; you can average them.
Disk: This is the hard drive, where all your files are stored. Heat can have a significant impact on the health of your drive, so this is definitely worth taking a look at.
In general, I recommend looking up the manufacturer's specs for the component in question (all components are different) so you know what the acceptable temperature range is for it. Don't be worried unless you exceed that range.
All that said, Macs are pretty smart about managing heat. If it really gets too hot, it will turn off automatically. That doesn't mean that you can't damage your computer with heat at all, but it does mean that you generally shouldn't have to worry too much about heat issues.
Your trip the Genius bar was too focused in my opinion. I suggest you make another appointment and lay out your case above, but this time, don't mention the memory tests, but that the problems showed up when you upgraded the RAM. Refer them to your old case number (tack this visit on as a follow-up).
Tell the Genius on hand that the problems still exist and that they need to do further testing. As it stands, your Mac is unusable. Additionally, tell them to make note that you have tried a fresh re-installation and it did nothing to help (don't mention that things ran fine for a day or too, stick to the current state of the system), which will rule out any software problems.
Ask them on what they suggest and follow it (no matter how annoying it may be). If it doesn't fix it, call them and do another follow-up. If they tested the RAM and it passes, then it might be the logicboard.
In any case, do not visit them with "the solution." Let them find it through there own testing. Clearly, the logs containing kernel panics and all the problems you list will support your case. Moreover, their internal testing tools are much more sophisticated than what you'll scrounge up on the net through 3rd party apps (they actually have in-house diagnostic utilities for these occasions).
I would highly recommend that you stop spending your time diagnosing the problem and let them do it for you. That's what they're there for.
If after everything is said and done, and they still can't pinpoint a problem, you will likely walk away with a new Mac. Just focus on the problems you are suffering from and leave the diagnostics to the Geniuses and technicians.
You may want to downgrade back to the 4 GB for now. And it's of note that replacing the RAM and the hard drive yourself do not void your warranty for that particular Mac model.
PS: You don't specify if you took your notebook to an Apple Store and spoke with a Genius or some Apple authorized repair centre. I would strongly encourage you to deal with Apple directly. They'll not only treat you better, but likely solve the problem with less headache. My advice is tailored to them; you are totally on your own should you choose a 3rd party repair shop. To galvanize the need to deal with Apple, let me put forth the following. I had a friend who purchased a Mac mini. There were strange problems during boot up where it would occasionally load the keyboard set-up assistant. The machine was also sluggish and exhibited other unusual anomalies in its day to day. We took it into an Apple Store and the Genius tried to reproduce the errors. He couldn't. He also expressly stated that minis never required repair or servicing. But with all that, he elected to replace the entire unit and gave my friend a new mini. Took it back to his place and it hummed, no more issues. I doubt he would have received the same treatment from a computer store authorized to service Macs. Deal with Apple direct; I cannot stress that enough.
Best Answer
It sounds like the fall may have damaged the fan. That would account for the heat and the noise. It's also possible the heatsink got slightly dislodged, which would also cause heat issues (although likely more severe).
In any event, unless it's clearly an unrelated problem (like a firmware or software bug), this damage is almost certainly not covered by Apple's warranty, as it's the result of an accident, not a part failure. However, from time to time Apple has been known to waive the repair charges on incidents that fall outside warranty coverage, although it seems to be on a case by case basis, and only with a bit of luck.