It says very explicitly that it gets six with its six longswords(because that's what plural means). If you want to house rule into being able to clearly violate RAI(it's freaking six armed with 6 different longswords for a reason), then go for it. That's not RAW, though.
Sure it could. One punch with each hand per the way its RAW multiattack is written and RAI very clearly intends.
No. Maraliths do not have extra attack. What is is, is written like two weapon fighting is written with a weapon in each hand. It would be pointless to give it six hands and six weapons, and then mention attacking with all the weapons if all it needed was one for its attacks.
Not as it is written or intended.
Absolutely not. They used the word "longswords"
because she has more than one longsword. To use the phrase, "Multiattack. The marilith can make seven attacks: six with its longsword and one with its tail" would be
factually incorrect, because she does not have
one longsword, she has six. English does not make a distinction in this kind of phrasing between these things. If I tell you I have six cookie boxes and that "I intend to eat six of my cookie," you would look at me like I'd grown a second head. But if I say, "I intend to eat six of my cookies," you would have
no idea whether I mean eating all type A, everything
but type A, exactly one of types A-F, or any other potential combination.
Your description is neither RAI nor RAW. The general rule stands, you may use whatever weapons you like for doing the thing, and there is nothing specific about this which should indicate otherwise.
And yes, it is pointless in a
mechanical sense. Much as the trident is pointless in a mechanical sense, being a heavier, higher-training, more-expensive spear. But 5e does several things like that, which are mechanically pointless but
thematically important. Thematically, mariliths have 6 arms, and generally wield a sword in each hand. Therefore, they have six swords, though they may choose to attack with only one of those swords if they wish.