The most reliable method involves "sideloading" the Phoenix IPA file from a computer using a tool like Sideloadly or the legacy Cydia Impactor.
While unlikely to matter for devices this old, jailbreaking typically voids any remaining warranty. phoenix jailbreak
While jailbreaking is generally in the US under DMCA exemptions for smartphones, it does come with trade-offs: Phoenix Jailbreak The most reliable method involves "sideloading" the Phoenix
Yes. You can unjailbreak your device by performing a full restore via iTunes on a computer, as explained in this YouTube guide . What If I Am on a Newer iOS Version? phoenix jailbreak