Is an academic career financially feasible for me?

This is a difficult question to answer.

To address the specific points of your question:

I'm a nontraditional student interested in pursuing an academic career in the humanities.

If you were already a nontraditional student when you graduated, what were your reasons for pursuing this degree, and why do you feel differently about pursuing a Master's degree/PhD?

Also, how have you been spending the time since you graduated, and do you have anyone who could help support you throughout graduate school?

Essentially, my best-case scenario would be achieving tenure at an age where most people are nearing retirement, and the odds of ever landing such position are slim for anyone.

I think you've answered your own question. To summarize, the odds are stacked against you, and even if you were able to get a tenure-track position, you're income potential could be limited. Thus, chasing your dream comes with significant risks, long-term.

However, there's no other career that interests me to the same degree, and there aren't any lucrative options with my degree anyway. Moreover, I'll always regret not even trying to pursue my dream.

You might also be able to channel your passion and energy for this area of research in different ways than your day-job (part-time consulting, independent research, volunteering, etc.).

Also, I'd challenge you to look beyond your degree. I'm sure you have transferable skills that would be applicable in a number of fields/roles.

As was pointed out on your question on Academia SE, regardless of financial considerations, if you are unwilling to relocate it is going to be difficult to pursue an academic career, period. Financial woes will only make it worse, because they'll mean you won't be able to be picky when searching for a job; you'll need to grab anything available so you can pay your loans.

I agree with 0xFEE1DEAD that there seem to be larger issues lurking in the background here. If all aspects of the plan were solid and the only uncertainty were about the finances, it might make sense to focus on that. But it sounds more like there are many separate eventualities that all would have to work out just right for this to turn out well. Given that, it doesn't sound wise to take a large financial risk.

It's really important to emphasize that many more people receive PhDs than there are jobs in academia. The vast majority of PhD holders work outside academia. Unless you are truly at the top of your field, there is a significant amount of luck involved in securing a position in academia and successfully navigating the tenure process. Universities only open up so many postdoc or tenure track positions, and there may not be any open when you're ready to apply. If they do open one up, you will likely be competing against several similarly or better qualified applicants. As you move up the ladder, you need success in teaching, recruiting students who eventually graduate successfully and (most important!) securing grant money to fund your research. All of these involve some elements that are outside your control.

In short, it's a huge gamble to take so you should actively research what your backup plan would be if you need to pursue a career outside academia with your degree. Are there jobs in your area for PhD holders in the subject area you're interested in? If so, do they interest you? Do they pay well?

A second point to consider: a PhD student is basically a research trainee with some expertise in a particular field. When you graduate, if you get a job where many PhD holders are employed (academia or industry), you will basically be at the "junior researcher" level, not at the top of the field.

So you should also make sure your expectations of autonomy align with the reality of the work you will likely be doing.