@Jason D. Only a judge can pierce the LLC, but I understand what you're saying. That's what I meant by "prove useless". Again, it boils down to how well the LLC is operated. In many cases people just set the LLC up themselves without legal help or they use a template on LegalZoom. If the LLC is not set up correctly or operated incorrectly the veil could easily be pierced. You're correct that a multi-member LLC doesn't get pierced as often.
Regarding the debt guaranty, it's not all that different from the LLC holding the note and you personally guaranteeing the debt. Many banks make LLCs have those arrangements, especially if the LLC doesn't have much history with that bank. Me personally being a liable party on the debt doesn't imply mismanagement of funds. In this case the LLC owns the property, and the LLC pays the debt.
I guess another way to think of it is let's say I hypothetically buy a house from someone with a Subject-To agreement. He deeds the home over to me, and I take over his existing mortgage payments. It's an a fairly common arrangement when it's necessary. That doesn't mean that I'm co-mingling funds with this seller just because his name stays on the note. Similarly, my LLC funds wouldn't be co-mingling with my personal funds.
The only legal implication I can see with being the guarantor on the loan is that I do not have protection from that specific liability. I am personally on the hook for that debt regardless if I have an LLC or not, and that is completely fine with me.