when does glowstone damage deal, at the end of enemy's turn? or before scoring, and if they send it back, can i re-empower it and send it back again

Yes — the damage hits at the end of the enemy's turn, during their Ending Step (Rule 317.1.a), not before scoring. Glowstone's trigger is "At the end of your turn," so whoever controls it when their Ending Step arrives eats the 5 to all their units. Hold scoring already happened at the start of that same turn (Beginning Phase), so the damage always lands after that player's scoring for the turn.

After the trigger resolves, the Expiration Step cleanup kills units with lethal damage (Might 5 or less) and then heals everything else — the damage is gone by the next turn, but anything that died stays dead.

And yes, you can ping-pong it. When the enemy activates the send ability, they pay "Disempower this, [T]" as the cost, so it arrives at your Base exhausted and unempowered — you can't re-send it that same turn. But once your Awaken step readies it, you pay [A][A] to re-empower it (it's "not Empowered," so the ability is legal) and fire it back during your Main Phase. No per-turn or per-game limit; each cycle just costs the sender [A][A] + exhaust, and if a player does nothing it self-destructs and deals 5 at their own Ending Step.