I'm not sure what did you mean about inserting full basic 10 into Xemu. Xemu/M65 is a hardware emulator, so it can run anything you 'feed" in, you only need the ROM image, much like the hardware that you can replace the ROM chip for sure. It's not like the "Hi65" which is basically a project being a "BASIC 10 emulator" without emulating the hardware. It's another question that - I think - there is simple no such thing "finished BASIC 10" ROM at all on our planet. Surely, it would be ideal somebody to disassemble existing BASIC 10 implementations from various ROM images with trying to bug fix + complete it, that wouldn't be an easy task though, but it's nothing to do with Xemu, but with BASIC 10 in general. Actually, Xemu has multiple emulators, now just thinking of its pure C65 emulation, it uses a ROM image file, you can specify any ROM image for it. For the M65 emulator, it emulated M65 more closely, eg you need an SD card image as the M65 needs an SD card, with the ROM image on the file system, so M65 kickstart can load it, like the "real" M65. It's up to you what kind of ROM image you can place on it (it's not even possible to put BASIC 10 ROM or any Commodore ROMs into it by default, or including it in Xemu, as it would be a license problem).
As for memory, the question is more about the context. Mega65 is more than only a C65 re-implementation it has its won ways to extend the idea of the C65. M65 has 28 bit address space, that is, basically 256Mbytes in theory. Many of the original C65 hardware elements are enhanced in M65 to allow to address the whole addressing space, including the DMA engine, CPU's support for MAP'ing memory (CPU memory mapper) and also new additions like zero page based linear addressing mode. Surely, 256Mbyte is a design information, still it depends on the actual implementation (future PCB for M65) what it can offer to connect things to, at which speed, and what amount of RAM it will hold by default. It's hard to say what C65 supported (now only C65, not M65 ...) as with eg REC (ram expansion controller) seems to have plans (just reading various preliminary documents, or even c65manual.txt), different versions. Honestly, I don't think - in any topic - we can say what C65 supported, since even existing C65s have different capabilities in many aspects, it was not a finished project at all. This is even true for the ROM/BASIC-10 btw ... Some of the ROM images you can find implements features simply does not work with other ROMs either giving implementation error, or not even known giving SYNTAX ERROR, or simply buggy, etc.