Your own vehicles and your group vehicles are safe, they wont get removed because of inactivity. (If you bought the group vehicles it's fine, 'cause if you lose official/recognized status for your group you will lose (some of) your group vehicles which are sponsored by HQ)
About the legal deals, yes if you both agree on a legal deal about your property (I suggest you to take some screen shots, just in case), your friend is forced to give back your properties as soon as you ask him to do so, because scamming on legal deals (regarding vehicles and properties only) is bannable.