(Due to some financial issues with him in the past, I'm not willing to cosign for him)  Would a rental agreement with the roommates suffice? 

Hopefully that upfront diligence plus a healthy contingency budget would suffice to keep me out of that scenario, otherwise I would foresee a guaranteed loss, or underperformance at the very least.

My wife and I are starting our journey with our first STR and I wanted to ask: Should we use an LLC for our Airbnb, or would an umbrella policy on top of our current insurance policy suffice?

and would that suffice?

I could go on and on, but suffice to say the devil is in the detail, not in the high level split.

REI investing for almost 20 years, owning a wealth management company for 22 years, suffice it to say there has been a lot go data/spreadsheeting/tax analysis/meeting with my team go into this overall decision to sell the entire portfolio at once.  

I'm sure they also told you their systems will protect against aggressive attorneys and creditors and make you invisible so nobody will ever dare file a claim against you...it's all a money grab.If you are a sole proprietor I am putting my money on the fact a single member LLC in the state in which you live/own real estate will suffice.

So far I have been providing a fixed amount that should suffice for a few days. 

What starting capital would suffice? 

At the very least, plenty of street parking would likely suffice.