Hi. Wow, that's a lot huh. I'm not an expert but I personally would not let a newly, older, already questionable, adopted dog be free roaming with visitors around. You know nothing of how the dog reacts to specific situations. Quite honestly, I don't let my current 1 year old baby free roam with visitors either because 1.) She is too young & big 2.) She can't always control her excitability (in positive & negative ways). 3.) She is still learning what is acceptable behavior. 4.) She doesn't know our visitors well enough for my taste. I could probably go on. My older dog has proven himself to be very trustworthy, so he'll be free roaming when people are over but I always supervise with kids involved.
If your dog is already showing signs that she can be triggered into over reacting, then take that as a warning. You can definitely work with her but you need professional help if you've never worked a dog like this before.
No one online will be able to tell you what caused the dog to go after this particular girl. We would have needed to be there, and even then we might not catch what the exact trigger was.
Besides professional help, I would also recommend a very NILIF, every day life style for your dog. Some dogs need it more than others, yours might do better with it. When you have a new dog coming into your home you need to place very structured rules to start you off on the best footing. (I'm not sure if you're already incorporating this.) You can google nilif if you don't know what it is.
As for food aggression, I find it hard to work with because "I" believe it can be learned or genetic. I've been working with dog on dog food aggression w/my own girl since she was a little puppy. I got her at 2 months so I highly doubt it was learned in her case. Here's a post where I mention what I do with her-
dog agressive with certain foods
My girl continues doing better but she also has set backs. I only test her when I am around and fortunately, she listens to me when things start going south and I take control of the situation again. It's a slow progress but I will not leave my dogs unsupervised with food involved.
P.S. I wouldn't lock her up when company is around but instead leash her to you while she works through her issues.