HAHAHA I'm laughing...
...because I got a 2 on my last exam, becouse someone exidentally counted 1 more point for me

On to topic:

Yes time can be altered, it depends on your speed, and on gravity, so physical time is also subjective time

If in LDs we use conceptual thoughts then I also think that we can "live" out more time compared to how long we actually dream in RL

About the brain overflowing with memories: I think our brains are smarter than that (at least mine is, I hope ^^), we probably sort by what is important and what is not and we forget some things so we can take in new expeirences

And for the topic starter: Maybe we already lived an eternity in a dream, it's just... we don't remember it? So how could it feel to experience an eternity in a dream? If you know it is a dream, you can surely wake up yourself at any point, even if not by will. And if you don't know it is a dream but you fully experient it, than watch the Matrix
Also I simply don't think our mind would die by experiencing an eternity