Hello @bethj, From what you described it sounds more like you may have a periodontal abscess instead of an abscess forming at the root area.
With periodontal disease, a ‘pocket’ can form where the gum tissue is no longer tight against the tooth. That gap can collect food debris and something as minor as a popcorn hull can get trapped and eventually cause inflammation and swelling. If that pocket becomes infected with pus it’s pretty painful and nasty. You may have had that drain overnight on its own overnight…explains the nasty taste.
https://www.healthline.com/health/gum-abscess
Usually if it’s an abscess at the root of the tooth, which would be what you had before in that tooth to warrant a root canal, it wouldn’t show up as a painful swelling around the gum line of the tooth. That’s more of a symptom of a periodontal abscess. If it was a root abscess that infected, it should have shown up on the x-ray.

I’m not a dentist so I can’t diagnose a condition, but I’ve been in the dental field for 30+ years and have experience with what you’re describing. If it is an abscess it would definitely need to be treated in the method the emergency dentist suggested. However, if the bone/gum health around that tooth is lacking then a new root canal and a new crown wouldn’t really be the best option.

May I ask when was the last time you had your teeth professionally cleaned? Did you have any discussions with your dentist about having periodontal disease? If there are pockets around your teeth, this can happen repeatedly. There are deep cleaning and reparative methods to help restore the gum tissue to a healthy level though so it may be worth postponing this appt to get a second opinion first.
Do you have a regular dentist you see? Could you get in to have a second opinion?