l'd still want to ask a confirmation for a few situations. Firstly, Glais Mejicula can replace shield breaking by discarding your hand cards. Does double substitution rule prevent you from putting cards like Arque Delacerna into the battle zone instead of discarding them (with Glais Mejicula) during your opponent's turn. Also, has double substitution rule always been applied, meaning that this rule wouldn't only apply for OCG format?
Secondly, can a duelist discard Arque Delacerna into the battle using Gigastand's ability: "When this creature would be put into your graveyard from the battle zone, you may return it to your hand instead. If you do so, discard a card from your hand". In this case, discarding could be interpreted as a separated case but it is still directly dependable on substituting Gigastand's placement to graveyard with returning it to its owner's hand and then discarding a card. Phrase "if you do" makes it more clear that you have to discard a hand card if you decide to return Gigastand to your hand instead of putting it into your graveyard.
These questions have risen in our local DM community. Thanks in advance for any help.