Will i be detained, or can i leave and go into home isolation and get a retest the next day?

You won't be able to get a retest the next day.

 

As to whether or not you'd be allowed home isolation this seems to vary by locality.

 

If you suspect you have (or have been exposed to) COVID you might do well to do a home ATK first before getting PCR.

If its a 72 hr test before a flight, result the same day, why not a test the day after giving a chance to get the flight?

try to test at your destination, some allow it within a few days after arrival (on condition you are fully vaxed).

In the other countries you can self-isolate. In thailand it's a lottery between hospitel from around 30k for 10 days and home isolation.

This several days delay might allow for your test to turn negative, if you suspect you had recent infection.