Wow ok. So many things are messed up here.
It's not something I would usually start with but: polish up that resume before continuing reading this answer and apply for any other SEO/marketing position in your area.
Once you've done that, realize that you were hired for the job you signed for. Not a whim of your boss to do something else, that is not what was agreed to in your contract. (Thus not what you're paid for, though you could send him an Invoice for work outside of contract I suppose - quick way to really be fired I'm assuming).
already doing all the SEO, Web Analytics, Business Development, Data
Management, 3rd Party Management, and Paid Marketing.
The fact you're already doing all of this and on top of that have managed to learn enough about programming to at least fix a few bugs in a system/website makes you a skilled employee. One capable of learning on the job, do what's needed. Your current boss is not recognizing this, but you'd make a very valuable employee with your mindset of learning all the things for the job and then some related things.
Your SE profiles show an affinity with Magento. Even on SO Jobs you should be able to find a few companies offering remote work in that.
Sorry, this turned into career advice / opinion. But as a web developer myself, I've seen this happen and have had this happen. The only option is to leave. Even if you manage to get a concession on something now, the next thing is only 2 weeks away of becoming an issue.
I have considered trying to find a web development mentor and paying
him/her to coach/help/guide me or potentially face unemployment. And
as insane as it sounds I have even considered just hiring a freelancer
and paying him/her myself to do the work just to get my boss off my
back and not have to worry about loosing my job or working 80+ hours a
week under even more stress.
Yea... don't do this. If your boss wants you to learn a skill, he/she has to pay for it. It's that simple. You were hired as you were to do a agreed upon tasks. This is precisely why people want education with their jobs, paid for by their employers.
Hope this answer can help you out.
Final kick in the nuts: today it's learning a bit of web dev to fix more bugs. Tomorrow you're doing devOps because AWS sign-ups grant 2 weeks free server hosting. GL & HF.