Say if somebody attacks master mold, does he have a chance to stun them back? seeing he can't attack?

he can't actively attack, but his ATK value would still be compared to the DEF value of anyone attacking him. therefore, he has the chance of stunning them back.

"Master Mold cannot attack."

Say I have Mastermold and Cyclops, Slim in play, along with a Marvel Team-up (X-Men and Sentinels). Could I declare a team attack with Cyclops and Mastermold? I know he can't attack, but can he be declared as an attacker? Does his game text only matter when the attack is checked for legality? If so, Cyclops would officially be team attacking, and would give himself the +2/+2 bonus.

"Master Mold cannot attack" is pretty self-explanatory.

In terms of game mechanics, all proposed attackers must be able to legally target the intended defender in order to declare an attack.

Since Master Mold cannot attack, you are prevented from even trying.