iPhone 8 battery swelling

Now you know why the battery stopped working. I don't think the delay would have made much difference to the outcome, fortunately the battery has not ruptured yet.

Be aware a lithium battery which goes from working to mega swollen in 2 weeks is at risk of rupturing if the swelling continues to grow. Ruptured lithium batteries have a reputation for catching fire, so please put the defective phone somewhere safe and fire tolerant before you turn in for the night.

When you go to your Genius Bar appointment I suggest you spend a few minutes looking at the contenders for a new iPhone. Depending how much damage was done when the battery forced your phone open it could be unrepairable, or you might consider the quoted cost of repair is uneconomic for a device which is roughly 4-5 years old. If you think of it as "new phone with new warranty" it might feel a little less painful.

This is not a known Apple problem, this is a known Lithium Polymer battery issue that affects all devices using them. If the are worn out (health is less than 80%) or have failed they will swell. This is not covered by any makers warranty unless it is a shorted out battery and occurs during your warranty period and is not due to any damage to the phone.

You should have had the battery replaced when the problem first manifest. It’s Possible that the phone will not be repairable.

Sorry, you said you waited two weeks in your post.

It is a known issue like tires that have worn down tread often have blowouts.