I made a wallet on Base first - then used that secret phrase backup in the AirGgap to add a wallet - are they designed to work together that way? - it seems to have worked. I am afraid of migrating my coins then having them be forever stuck in limbo for some stupid reason. Thanks.
Your seed / private key is valid for any wallet software.
Yes, they are. The Base aepp supports using AirGap Vault on a separate device in both iOS and Android native apps. It supports AirGap Vault on same device in iOS. It will also support AirGap Vault on the same device in Android soon, as well.
As @r0b0t0 pointed out above you can always backup your recovery/seed phrase and import it into another wallet.