So last night we played rebel smugglers vs Bounty Hunters. During one round Dengar placed BLEED and STUNNED on Solo, followed by WEAKEND using his Payback card after he used Return Fire.

I could not find a quick answer online as to whether STUN would keep him from Returning Fire and also doing his new end of round attack. Whereas Bleed says it only takes effect "during your activation", stun does not have that caveat.

We decided he could still take those 2 attacks, but in retrospect, I think he would not have been able to attack either time due to the stun.

no you cannot, both Return Fire and Rogue Smuggler state interrupt to perform an attack, and Stun clearly states you cannot attack.