A sole proprietor can open a Solo 401K.

I won't go into my perspective of why this is the case unless you want to hear.The main takeaway here is that only you can understand and decide if you are wanting to truly be an entrepreneur and a 'sole proprietor'. 

Are there any RE agents whose long-term goal isn’t to remain an agent and/or own their own brokerage and have been successful operating as a sole proprietor?

As a sole proprietor, your tax ID is your SSN.If you prefer to not give your SSN to third parties, you can apply for a sole proprietor EIN.

So to recap; If these were purchased in your name, it seems like a separate personal account would maybe work - basically you're acting as the sole proprietor-ish in this case, and while missing on the tax advantages of an LLC and the pass through income, it's all coming back on your personal taxes anyway is what I mean.  

I am to understand the 1099'd and sole proprietors are eligible and the requirements for these businesses are still being determined.

I’ve heard that if I’m sued the plaintiff can potentially have access to funds and other assets acquired as a sole proprietor, and in some cases, even if my REI assets are held in a single LLC.

This requirement is especially common if you're holding the property in an LLC, sole proprietor they just sue you directly.The security deposit becomes yours only after non performance of the lease in some regard.

Even if your next task is to learn about mortgage types or to learn how home insurance works or to decide on LLC vs Sole Proprietor ownership (<--- Hot Topic).

In the rare case someone does object you can assure them, in writing if needed, that you will be the proprietor of any LLC formed, good luck!