If I flip Prowl to alt mode and give my Cars Bold 2, but later in the turn I flip one of my Cars so it’s no
longer a Car, does it keep Bold 2 until end of turn?
[Source: WotC]
Yes. Once the bonus has applied, the character will keep Bold 2 until the end of the turn.
Prowl’s ability says "When one of your Cars is KO’d -> Repair 3 damage from one of your other
characters." Does "one of your other characters" mean other than Prowl or other than the Car that was
KO’d?
[Source: WotC]
It means other than Prowl. Prowl can’t repair himself.
Does Prowl - Military Strategist (bot mode) ability trigger when Prowl - Military Strategist gets KOed?
[Source: WotC]
No, it doesn’t. In bot mode, Prowl is not a car.