I know stringed instruments very well, and my opinion is that your mandolin needs to be cut loose and sent on to another home.

Basically, the neck is bent. Yes, there is a truss rod that can be adjusted that will take some of/most of the warp out of the neck, but the nut and tuners are still going to be a problem. An enthusiastic player could put the work into restoring it and getting it working properly again (at which point something else will break), but you won't.

What you need is a "player" instrument. Something that just works, and lets you plink away at it any time you want. Doesn't need to be pretty or fancy in any way at all. Just something you can hold in your hands, stays in tune, and has a reasonable amount of sound output.

Because you aren't a "player" yet, you need something you can just twiddle on whenever the feeling strikes you. Pick it up, pluck a few notes, see if you want to pluck a few more.

If every time you pick it up you need to adjust the tuning, or the nut, or you know you can't play in tune above the 6th fret, then you won't get any pleasure out of twiddling, and therefore your twiddling won't become playing, and playing won't become a passion.

And, one last thing: When you replace this mando, store the next one you get properly. Don't hang it up in a sunny window, or lay it on a shelf in a damp basement. Put it in a stand or a wall hanger, where it is out of direct sunlight, but easy to reach, and in a room where the temperature don't vary crazily. (Don't hang it over the fireplace.) Wood flexes with moisture and temperature. When stringed instruments flex, they crack, warp, go out of tune, etc. And don't leave it in the case. Cases are for traveling, not for storage, and they prevent you from picking the instrument up on a whim and widdling away at the strings whenever the passion strikes you.

Good luck!