No, as long as you don’t uninstall Spike, you can reinstall it as many times as you want without losing your settings. If you wish to uninstall Spike make sure you first do a backup of your settings or database in Spike’s Settings -> Maintenance.
No. CGM transmitters can only transmit data to one app at a time. Spike has all the features (and more) of the official apps so it’s recommended that you uninstall them and use Spike exclusively. If the official app has an Apple Watch complication you should uninstall it as well.