Dui, ni shuo de dui
Yes, it can make quite a difference, but it will still have a tiny bit of flex, no matter what you do.
I honestly don’t think so. The MPCNC is not designed to face crazy accelerations anyways, so inertia is not really a big concern.
The biggest issue will be to attach your tool on the Z axis, some hardened tubes cannot be drilled, so You’ll have a hard time to attach anything. I had this problem on my machine, which uses hardened, very thick tubing.
Yes, it it best to add to the outside diameter whenever you can. However, it cannot be done here, so the only solution left is to increase Internal diameter, even if it is not the best option. Still better than nothing. There was a link to a strenght calculator on the forum, increasing the inside diameter actually increased strenght quite a lot, even if it was not linear. At some point the deflection becomes too small to really matter.