If there's nothing for Harold and Bob to legally enchant, it remains in its owner's graveyard instead of returning to the battlefield.

The cards are milled all at once, which means abilities that trigger "whenever one or more nonland cards are milled" will trigger exactly once as long as at least one nonland card was milled.

If you control Harold and Bob, you'll return it to the battlefield when it dies while it's a creature. It doesn't matter who owns the card. You'll choose which Forest you control it will enchant as it returns to the battlefield.

If a nontoken permanent that's a copy of Harold and Bob dies while it's a creature, it will return to the battlefield as an Aura with only the abilities granted by the effect that returned it to the battlefield. It will also retain its name, colors, and supertypes, but it will be an Aura enchantment with no other types or subtypes.

If a player has fewer cards remaining in their library than the number of rad counters they have when the triggered ability resolves, they'll mill as many cards as they can.

Any effects (such as proliferate) that interact with counters a player gets, has, or loses can interact with rad counters.

A token that's a copy of Harold and Bob won't return to the battlefield from its owner's graveyard.

Rad counters are a kind of counter that a player may have. They're not associated with any specific permanents.

Rad counters don't go away as steps, phases, or turns end. They only go away when an effect instructs a player to remove rad counters from themselves.

There is an inherent triggered ability associated with having rad counters. This triggered ability has no source and is controlled by the active player. The full text of this ability is "At the beginning of the precombat main phase of a player with rad counters, that player mills cards equal to the number of rad counters they have. For each nonland card milled this way, that player loses 1 life and removes one rad counter from themselves."

Keep track of how many rad counters each player has. Potential ways to track this include writing them down on paper or using dice, but any method that is clear and mutually agreeable is fine.

In a game using the shared team turns option, such as an Archenemy or Two-Headed Giant game, the inherent triggered ability associated with rad counters triggers once for each player on the active team that has rad counters. Each instance of that ability is controlled by one of those players.