First, note Roy Harper's text:
Activate, KO a resource you control -> Stun target character with an ATK less than Roy Harper's ATK
Roy Harper cannot activate until the requirement is met.
Until Roy Harper's ATK is greater than Killer Croc (whose base ATK is 7), there is no issue.
Next, we look at the Attack step:
You propose (note, you do not attack yet) an attack with Killer Croc, targeting Roy Harper. You do not exhaust yet and will not exhaust, nor become an attacker, until both of you pass on an empty chain. You pass.
Opponent exhaust 4 Teen Titans characters in succession, letting each resolve (you both pass repeatedly). Roy Harper's ATK is now 12, greater than Killer Croc's. Roy Harper activates for his second effect, targeting Killer Croc. (note that the attack is still in the proposed part. Killer Croc is still not an attacker). Both players pass and Killer Croc is stunned. Both players pass on an empty chain and the game sees that one of the proposed attackers is illegal and the attack is cancelled.
Now, let's look at a slightly different situation. This time, Killer Croc proposes an attack and both players pass allowing Killer Croc to exhaust and become an attacker, with Roy Harper becoming a Defender. First, the continuous effects apply (Fear and Confusion). This doesn't go on the chain, so Killer Croc is an 8 ATK. Next, the triggered effects go on the chain. In this case, both triggered effects are controlled by the attacking player, so that player determines the order they go on the chain.
Assume the player chooses:
1. Killer Croc ATK increase.
2. Defender (Roy Harper) is exhausted.
At this point, the attacking player has priority and may add an effect or pass. The attacking player passes. The defending player has priority now and may put an effect on the chain (note that the effect number 2 has not resolved yet). The defending player chooses to exhaust a Teen Titan's character 4 times before passing priority back to the player. The chain thus far looks like this:
1. Killer Croc ATK increase
2. Exhaust defender (Roy Harper)
3. Roy Harper +2 ATK
4. Roy Harper +2 ATK
5. Roy Harper +2 ATK
6. Roy Harper +2 ATK
Assuming no one has anything else to add, both players start passing consecutively until we finish resolving down to 3. Roy Harper now has 12 ATK (Greater than Killer Croc's 8 ATK). The attacking (primary) player has priority and passes. The defending player activates Roy Harper, targeting Killer Croc. This is legal because Roy Harper's ATK is greater than Killer Croc's. So the chain is now as follows:
1. Killer Croc ATK increase.
2. Exhaust defender (Roy Harper)
3. Stun Killer Croc if Killer Croc's ATK is less than Roy Harper's.
Both players pass and Killer Croc is now stunned.
Both players pass again and Roy Harper is attempted to be exhausted, but is already exhausted.
Both players pass and Killer Croc's ATK increases by 3.
Both players pass again and the attack goes to conclusion, finds that there is no attacker and the attack ends.
As for the Tower of Babel, it depends on when it is played. Once an attack is deemed legal (attackers have exhausted), nothing can make the attack illegal. The game doesn't care if they don't share a team affiliation after they have become attackers. This is the same idea as moving a defender behind another after the attack is legal, even if at least one of the attackers doesn't have flight. The game doesn't care about that. It checked for legality when both players passed on an empty chain in the proposal step.
Tower of Babel applies only to characters, which are on the field in the front or support rows. It does not affect cards in the hand, resource row, deck, or KO pile.