Wellcome to the boards
1.) Anima dosn't distinguish between attackers but gives penalties based how often you have to defend: For the 2nd defense you get a -30, for the 3rd a -50 for the 4th a -70 and for any defense after that a -90. These penalties are not applied if you use supernatural shields to defend.
2.) a +75 attack(single) +100 limited dodge ki technique gives one of your attacks a +75 attack bonus and all your defenses a +100 dodge bonus - thought you won't be able to counter attack on a successfull defense.
3.) Sadly, you can't combine any effects from two or more Ki techniques unless all are Combineable (a advantage added with the book Ki Dominion), so you can't asing the +75 attack bonus to any of those 3 limited attacks but only to any one normal attack you could make.