Installing LEDE/u-boot via brnboot web interface (without rs232)

I've only tried the NOR variant (especially for FXS/ asterisk operations, you do want the additional space that gives you), so I'll only provide a partial answer.

1: yes, mtdsplit being at work, subdividing the given mtd.
4: no, there is only one targets/lantiq/xrx200/u-boot-vgv7510kw22_brn/u-boot.bin, which covers both hardware revisions. The reason why the old OpenWrt wiki lists two, is because the newer revision required a newer u-boot version/ snapshot (newer than CC) than the older. Current u-boot versions (LEDE) cover both variants (reality diverging from (OpenWrt/ wiki) documentation confused me quite bit while doing the initial flash).
5: afaik no, the kernel (DTS) provided cmdline override, hardcodes nosmp - you need to patch DTS/ recompile/ flash to change that (I'd be rather interested in a VGV7510KW22NORSMP/NOFXS) variant myself for different use cases/ devices without having to build it twice (using MULTI_PROFILE/ PER_DEVICE_ROOTFS).
6: if by “Annex J” you mean pure ADSL2+ && Annex J operations, the default firmware preinstalled by LEDE should be fine - you only need to update it if VDSL2 && vectoring is required (if you're in doubt, updating shouldn't hurt either).

Using VGV7510KW22NOR, I haven't observed any oddities so far on two devices (one as modem (uptime >4 weeks), one as fxs gateway (shorter uptimes, as I'm still tweaking the image/ configuration), although neither of them are in real production so far).