The antenna connector is probably the most difficult one to attach. just keep at it, gently, and it will eventually seat. I have the best luck using my fingers instead of a tool, so I can feel if I am getting the alignment correct. I have big hands, so I always seem to have trouble with antenna cables.

looks like it should be low profile enough to fit in the same space as the regular HDD

The fan should spin at max RPM if any temp sensor is not detected, as a failsafe. Is something interfering with the fan mechanically? The SSD should draw less power than a regular HDD, so I am wondering if this might be mechanical and not electrical. You can run the unit without reassembling it to see if a cable or the SSD housing is interfering. I can't image the SSD housing is causing a problem, but you never know.

You can run with just one RAM module, but you will get the best performance by installing a matched pair (same size, same manufacturer). This allows the Mac to interleave the RAM, essentially doubling the speed the CPU can access memory.

Do you get a boot chime? If so, shine a bright light on the screen after powering it on and look for an image on your screen. It may just be that your backlight has failed. Optionally, connect an external display and check for video. Also try booting with no battery installed - sometimes a bad battery will pull down the entire power circuit.

I would do fans, then hard drive, and end with RAM. You don't need to remove any of these items to replace the others, but you do have to remove the left i/o board to get to the left fan, so might as well start with the hardest step first.

Sounds like possible liquid damage

Your internal hard drive has definitely failed - try booting the computer from DVD after disconnecting the hard drive cable from the logic board. Do you have the original restore discs for this computer? It requires 10.6.3 minimum, and a Snow Leopard retail DVD would not necessarily boot the computer. There are two revisions of the Snow Leopard DVD - 10.6.0 and 10.6.3. If you have access to another Mac, try creating a Lion or Mountain Lion DVD to test with. Sleep mode would not cause a hard drive failure, as the drive heads are parked just as if the computer was powered off. A drop or strike, however, could be the culprit. If you have AppleCare on this computer, I recommend taking it to an Apple Store or Authorized service provide and let them deal with it for you for free...

I found this pic for you, hope it helps: Won't power on even after replacing logic board.

I would start with backing up the phone, then performing a full restore of the iOS. It could be corrupted software and this is, unfortunately, the least invasion step. If the overheating continues with just the iOS, inspect the dock connector and headphone jack liquid immersion sensors - if they are red/pink, then moisture has gotten into the unit. Use a flashlight to aid visualizing the sensors. In my experience, it takes _a lot_ to set them off in the iPhone4 - I had a coworker drop her's in the toilet, she fished it our immediately and got it to me fast. The sensors were still white, inside and out, then phone continued to function just fine after I disassembled and dried it out. If you can, don't restore your data from the backup yet - it is possible there is corruption in the data and leaving it off for a few hours can help with issue isolation. If the phone appears to be working properly with just iOS, then restore from backup (right-click on the iphone in the left column of iTunes for this option)....