You are in no way immune to altitude sickness, no. Physiologically you do not change. Some people do seem to acclimatise faster than others, but no matter how many times you summit Everest, if you got plonked on the summit with no prior acclimatisation you'd die.
I agree with all the stuff until the nutrition section: I can't see there being any point in carnosine or AA's being consumed at altitude because you should never working hard enough to build up lactate/acidosis. Altitude is all about endurance and you should never be working anywhere near your lactic threshold. I agree that while in training if you're doing hard intervals or squats etc. then a turkey sandwich or carnosine supplement might be a good idea, but I can see no point in it at altitude.
They don't make much sense to me either and this is something I claim to know something about! I don't know where Multimat get their tog values measured, but I'd be very surprised if a small difference in foam type can break the 'thickness is proportional to tog' rule. Maybe the foams are a totally different type? I would go for the Backpacker - you are doing this to save weight over a thermarest so go for weight saving. 100 g in foam is not going to make the same difference in warmth as 100 g in down (ie a sleeping bag).
For bivvying at 2000-2500 m in mid-summer in Switzerland I'd carry a sleeping bag good to 0 °C (250-300 g down, roughly), a lightweight mat (eg. the Backpacker) and a down jacket for cooking, sitting in, and as a booster on the sleeping bag. That's been good to me down to about -5 °C.
If you're looking for a 'temperature rating' for a foam pad then that's flawed as it obviously depends so much on other factors, such as you and the sleeping bag. However, I think for summer use away from snow, there is nothing wrong with a foam pad. It's not as comfortable as a Thermarest, but that's the only real downside. I've recently got a Neoair Xtherm and that is the first pad where I've noticed a genuine difference in warmth versus other pads - you feel like you're lying on a heater!
Watch this space for a standard (like EN13537 for sleeping bags but hopefully not as bad) for sleeping pads. It could be on its way...