Well, I'm so happy that you do at least have a proper diagnosis. Because NOW it can be addressed.
In answering your questions above:
1. I can't speak for your Dentist or ER Doctor. I mean I know how you must feel. But from the beginning, the Dentist with the Dental x-ray, thought it was a wisdom tooth infection, right? Now the ER Doctor didn't see anything (initially anyway). And last, the radiologist looking more carefully finally determines they've all missed seeing the Parotid duct (saliva duct) clogged up until just most recently. I am glad that they've found the true problem however. It is NOT a frequent problem, is hard to see, detect, and diagnose, so with all that said, it's difficult, but at least it has now been identified. I'd focus on that & getting this swelling to decrease now...
2. At this present moment, I feel very confident to tell you that some of this swelling is from infection, and some is from a clogged saliva duct and gland. So, I would stay on the present antibiotic. You will get better. And you will get back to looking normal. Yes the swelling will go away totally & soon. The sooner you see someone who can get the stone out, the sooner you will begin to get relief and return to normal. I can't tell you specifics, but you'll feel almost instant relief once the stone is removed, and the swelling will subside, but understandably will take I'd imagine a week or so to begin to look more like yourself.
3. I understand your tachycardia concern/antibiotics. I'm assuming you addressed this with the ER Doctor. If he gave you a recommendation, I'd go by that. If he didn't you'll then have to use your best judgement. If it were me, I'd probably error with taking them (just don't be excessive, of course). Again use your judgement.
4. Here is the BIGGY - assuming the Radiologist and ER Doctor are correct, you need to get the stone out!!! Do as they told you, eating sour candy also things that are sour like grapefruit juice, etc., but these things are only possible helpers over a weekend. They have a 10% chance of working, so try them. But what you likely will need come Monday morning is to see an Oral Surgeon (dentist/specialist). So try on your own with sour things, possibly the antibiotics, etc., but you'll LIKELY need the services of an Oral Surgeon. They always go through the mouth. NOT the cheek. Often, they can manipulate it out, by palpation (working it out with mild hand pressure) alone. If no, they can open up the small (in the mouth)parotid duct a bit more, and pop it out! If it's going to hurt - he'll numb you, so don't worry about that.
BOT***** *****NE: DO WHAT YOU CAN TONIGHT WITH SOUR DRINK/CANDY. BUT COUNT ON SEEING THE ORAL SURGEON TOMORROW! Explain how this is an emergency situation/ER visits/clogged parotid duct/facial swelling. He or she will work you in one way or another, even if they have to stay late after normal hours. Again, as soon as they get the stone out - you will almost immediately begin to feel relief. The swelling will begin to subside, and you will be back to normal in days/ a week(ish) time frame. Quickly, by all standards. But, you've got to get the stone out - OK? So, if they didn't already tell you that, at the ER, do so immediately as possible on Monday! I'm sending this to you now. Please let me know how things go for you, alright?
I hope you get feeling better ASAP! ( -:
*PS: If it weren't Sunday, I'd get to an Oral Surgeon today. But, I think they are all closed. You could check around for after hour phone numbers on their websites though? But, if no luck, get in to see one on Monday- OK?