im using nvidia drivers also on a 2560x1440 monitor and im able to use all weird resolutions in games i played.
do you have example of game where it screws up for example? whats your nvidia settings conf? nvidia-auto-select?

The nvidia drivers report everything that is included in the EDID info from your monitor. The "scaled" modes refer to resolutions that run over the timings of another resolution. An extremely useful thing since you can configure your display exactly the way you want to. Unfortunately, xorg/xrandr does not allow all the things that the nvidia driver can do with viewportin/viewportout and you can't store these pseudo modes, making them unavailable (and misunderstood) in applications. In my opinion, this is something that needs to be addressed in xorg/xrandr.

Am I just going to have to throw out my 4k television because a majority of my game library chugs at its native res? Can't use the open-source drivers, doesn't play well with my 3D modeling software.
There's gotta be some way to jury rig the thing until some developer from wherever addresses the issue.

Is there a reason why playing games in windowed mode is not an acceptable compromise? Or am I misunderstanding and that is not working for you as well?

I do not find that to be an agreeable compromise. I find myself distracted easily by window titlebars and wasted screen real estate.

still you didn't list any examples. So its only some games that do not have some resolutions in their list? what about forcing resolutions via command line to the games? like for unity games -screen-height xxxx -screen-width xxxx, or tweaking the ini files etc. 4K is quite useless for gaming indeed, theres no GPU out in the market that can deliver desirable framerates on that resolution in many games. 1080Ti is the closest there is to push 4K, but imo still not there in terms of performance.

Funny thing about windowed mode. A lot of games will only report window sizes from the full screen resolutions list (Feral games for instance), which makes windowed mode pretty useless if your monitor doesn't cooperate.

Didn't really list any games because it just seems to affect all of 'em with very few exceptions. Seems like anything that isn't running through wine or is on the bleeding edge doesn't report alternate resolutions. Recently I picked up those free Amnesia games and neither reported anything but native res, you can tell it to run at a custom resolution, but the games'll ignore that and just run it at my screens full 4k. Happens in most source ports I try such as: Quakespasm, Yamagi Quake 2, Eduke32, and OpenMW to name a few. If I tell games to run at a certain resolution in the console, it'll often just ignore it or break in some manner.

This is one of the few times I've been unable to get Linux to behave in the manner I wanted it to and I find that to be bothersome. Recently I spent several hours getting the Linux framebuffer to work so that I could display movies and pictures from the console without staring an X Server. That was really neat, seems like if I'm able to do something like that I should be able to do something trivial like playing Quake at a ridiculously small and unplayable resolution just for larks.

what about adding list of metamodes into nvidia's xorg config.

with one monitor something like:

Option "Metamodes" " 3840×2160;2560x1440,1920x1080;"

and so on to list all you need.

Did that, unfortunately didn't work. Adding a resolution like 853x480 allows me to switch to it with xrandr, but not in any game that has this issue.

that is unfortunate, i never experienced this issue with my 2560x1440 monitor. games list lots of resolutions i can choose from. though i usually just can't deal with anything lower than 1440, cause the games will be quite blurry with lower resolutions due to pixel scaling.

Never really minded pixel scaling myself, at least to a point. 480p in entirely acceptable when I'm couch gaming, much more acceptable than a low framerate.