Point 1, not sure. Looks like some intermitent problem.
Point 2, maybe there was some misscconfiguration or the CD was disabled...
A RAM problem can cause a boot failure at any point, it depends of exactly WHERE (at what address) the RAM error is situated. I've seen RAM failure cases when the boot process seems to proceede normaly, just to have the OS miss start and cause a restart loop. In other cases, it can cause random crashes and freeses, including some BSODs.
Total boot failure caused by faulty RAM only occurs if the fault is situated in the first few Kb of memory. In this case, you can't even access the BIOS configuration.
Your problem seems to be intermitent, making it realy hard to diagnose.
Try using some other RAM, or, if you have more than one stick/bank, removing some of it. If removing part of the RAM or replacing it solve the problem, you have a RAM problem and need to replace the defective RAM.
Try removing and replacing every easily removable component. DON'T touch the CPU, but remove and replace the RAM, every expantion cards, all power and data cables. Some component may be unseated or the contacts dirty or coroded. This process ensure that every thing is properly connected and any dirt/corosion rubbed away.
Try leaving the computer ON a few minutes, then perform a boot attempt.
If it works, it can indicate a cracked or broken weld of some component. In that case, the motherboard is no longer reliable. Finding the fault is near impossible as it may be located inside the casing of an IC circuit.
Point 4, you are right, your BIOS is working correctly and, at least, the low RAM is problem free.
Point 5, normal diagnosis procedure, good work on your part.
Point 6, if the MBR was corrupted, it would have been impossible to install Windows 7 without having the install process perform a full formating of the drive.
You are right that it won't affect booting from some alternate source.