[General Questions and Answers] K3n H1mur4:
Nexus 10 is the device you're looking for. Extremely (2560x1600) hi-res screen, no mobile data, heavy community support (it's a Nexus device, dude!), pure Android (for the same reason of the latter), around $400 on Play Store. Does not have MicroSD, and, from what I reasearched, USB OTG works read-only on non-rooted devices (but I don't think it'll be an issue for you).

Though not the same quality as Note 10.1's ones (which are made by Wacom, one of the best companies in this matter), there are capacitive-compatible stylus pens which will wok on any device (just google capacitive stylus and you're set). About KLP, all Nexus devices from previous generations (obviously, considering hardware limitations, like on official Jelly Bean for Nexus One and S) receive the update shortly after a new Android version is released. Now, all Nexus devices since Galaxy Nexus have 4.2 available: GN itself, N7 (which came with 4.1), and N4/N10 (which already came with 4.2). So, by getting a Nexus 10, you'll surely get KLP in a few days after the first device with it hits the market (maybe on the same day)

Here you get XDA-Developers'boards filtered by top devices. I think it's listed by per-board activity (though I'm not sure), so you can just look for the topmost tablets (it includes all current devices forums, smartphones, tablets, and other Android devices - Ouya is the topmost of all devices right now, i.e.) on this list. Anyway, the most recommended ones are Nexus 7 and 10, since, like all Nexus devices, they're developer oriented, so there are a lot of custom firmwares for both of them. Also, they pack state-of-the-art hardware (not that true for Nexus 7, but totally for N10) and have unbeatable price tags if you live in a place where Play Store sells devices (I know of USA, UK, France, Australia and Spain, but the list might be longer).

About Custom ROM status: from what I researched, all three devices have incomplete CM10.1 support, but One X+ is the closest to get full support (only audio quality seems to be an issue, and they're waiting for official 4.2.1 from HTC to come out to solve it). On Optimus G, the list is here. Here for Note 2 (I know it says i9300, but, in the Wiki, TeamHackSung says they issues are the same for GS3 and GN2), and here for One X+. You should note that on Note 2 and Optimus G, though, the releases aren't for the worldwide versions of the devices, so you should take a look if they're flashable on other device versions, if those are not for yours. Now the other questions:

About changing your phone, it's mostly up to you. All 3 devices have great hardware and, I think, will be able to get, at least, 2 more official Android releases after 4.2 (and even more community-based CFWs). Performance-wise, the Optimus G has the best specs: latest gen CPU and GPU (quad-core Cortex-A15, versus quad-core Cortex-A9 for GN2 and OX+ - for clarification, an dual-core A15 easily beats an quad-core A9, you can see this easily in comparisons about the original One X versus its AT&T version), along with 2GB of RAM (also available on the Note 2). The screen on the Note 2 has a better technology for contrast, but its composed of a PenTile matrix (it has 2 subpixels per pixel, opposed to 3 for usual LCD screens like the ones on OG and OX+), and, at the screen dpi of the N2, it's clearly noticeable. It's known that LG has put some software-wise improvements in its phone, one can see it by comparing, side by side, an OG and a Nexus 4 (which carry, concercing hardware, the very same screen), which, together with the better DPI and technology of the screen, results in better overall appearance.

So here's your answer: if you're looking for a more future-proof hardware, go for the Optimus G. HTC's only quad-core A15 phones released are those 1080p monsters (for clarification, any pixel density over 300 dpi [that means: any 720p screen of 4.89" or below] isn't noticeable by human eyes, so there's no actual need for 1080p screens [though I'm pretty sure we'll see 4k phone screens in about 2 or 3 years]), and Samsung has no quad-core A15 devices yet (on Android, only about 3 or 4 dual-core A15, but no flagship, which means almost no dev community, and even less custom firmwares available). For the imaging concerns, Optimus G also has the best screen, and some E973 versions has 13 MP shooters, which are better than both GN2 and OX+.