Ouch. I feel your pain. I am using Z lights at the moment and having no end of problems.
Have you done a firmware update on the bulbs yet ? If not install the updater https://www.lifx.com/pages/updates on a laptop or similar with wifi. The bulbs don’t need to be on the network to run. This works so much better than updating via phone. Ensure you have the latest firmware.
As I said if you are using Android and what you are saying seems to reflect that go into your mobile data and disable mobile data whilst you are configuring the bulbs. This helps a lot. I had real issues adding my Z units before I did the firmware update but once I did that they were much easier. (keeping them on the network is another issue)
I have also found that once I have manually connected to the bulbs setup wifi with mobile data turned off the phone remembers that and is much easier next time, that is the switching back and forth between wifi networks stops.
Check your router, make sure you are using only 2.4 ghz and not in the low or high channels and . If you can set a reserved IP address for each bulb and check that your router does not have too many devices connected to it
Having one bulb go down should not be an issue for any other bulbs, that is just weird.
You can easily change the group in the app by clicking on the settings icon in the bulb. if this is failing maybe firmware version ?
Raise a ticket with their support team. I hope you can get it all sorted out.
I assume you have seen these.