Sure, it's technically possible since we have kernel access, but the games aren't programmed to take advantage of it. It might make a difference in games which use multithreading (assuming those exist outside of n3DS exclusives) but other than that the games would have to be modded to offload certain parts of code to the extra cores, which might make it unstable and crashy.
Even that would take a lot of effort and would be game specific most likely, RAM is a different thing altogether, the game engine was designed to work within a certain amount of RAM and throwing more RAM at it doesn't mean there's a way to take advantage of that extra RAM unless maybe a game uses a large part of RAM as a cache, then the cache size could be increased which would theoretically speed things up a little depending on how often the cache is used. RAM will just limit what they can put into the game, it generally won't affect speed, it's not like the 3DS uses a pagefile. Stuff either runs, or doesn't depending on if there is enough RAM.
Arm processors have a maximum safe value I bet both systems are at that value. If you want to crack the kernel you could probably over clock but that would cut the life of the chip at least by half. I would maybe focus on a better scheduler or I/o clock speed is more or less nothing
Overclocking within reasonable limits shouldn't have a significant impact on the lifetime. It's only when you push it to the limit that you can decrease the lifetime of it due to more heat/overvolting. Overclocking a little doesn't actually make ARM CPUs run much hotter though, as long as you don't overvolt.
I wouldn't be surprised if the 3DS CPU was underclocked to save power like the PSP to be honest. Anyway, there's always a reasonable amount of leeway for overclocking CPUs from stock without shortening the life span significantly. Just have to pay attention to those temps.
You can't really do that, especially RAM. If a game takes use of all 256MB that the N3DS has, you can't really put that into 128MB on an old 3DS. You simply can't just magically make ram appear.
I think you got it the wrong way around. He's talking about o3DS games on n3DS, not the other way around.