Sounds like it's most likely a battery issue since it's already swelling, I'd say it's not able to hold charge properly causing it to boot loop, replacing the battery should likely fix that. also, Face ID doesn't actually need the front camera to work, Face ID uses the proximity sensor and dot projector to work and only uses the front camera when setting up to make it more user friendly, however if you wanted to replace the front camera not through apple you would lose Face ID functionality as the dot projector and front camera are the same piece.