1.) MATK buff should just be a straight buff, not sure how much it affects it. i believe the formula is about equal, but STR/ATK seems to be slightly more favored than INT/MATK probably because of the 0.5% weapon attack in renewal stats for STR.
2.) it should behave the same? remember grand cross is NOT buffed by card % dmg so it doesnt matter what type of cards (say archer skeleton on a bow) would do. A bow would probably be bad due to its lower attack and less than ideal size mods. (even tho they are decent compared to say a dagger)
3.) Grand Cross is about the same as it was in classic, but because everything else is better now, its kind of fallen behind its holy cross counterpart. While its not the ideal build, it can still be a fun situational build if you are built specifically for it. and it can still do good damage.
4.) they've talked about it in length before the server launch and i think it was about 70/30 that it should not be buffed. its weaker, but it doesn't necessarily need a buff to be viable. I think people should remember that not everything has to be "meta" to be viable. If you go all out and specifically build for most abilities that aren't total garbage (and grand cross isnt) it will do work.