Question 1: if someone jumps onto your table at a + count they will use up cards and you will get less hands when you have the advantage so it costs you money. If someone jumps onto your table at a - count it uses up cards and you will get less hands when you are at a disadvantage so it will save you money. If someone tries to jump on your table at a + count you can ask them to "please wait until the end of the shoe so you don't mess the cards up" and hope they stay out. If someone wants to jump onto your table at a - count welcome them in.
Question 2: Index variations specify you make a play either above or below a certain number. 5,6,7 etc are above +4 so a play at +4 or greater would be at 4,5,6,7 etc. A play below a certain number, for instance, would be to hit a stiff at or below a certain number but stand above that number. Hitting a hard 14 against a 2 for me was at -3 or lower so I would hit 14 v 2 at -3. -4, -5, -6, etc. Hitting hard 12 against a 3 for me was stand at +2 and above and hit below +2 so I would hit a 12 against a 3 at 1, 0, -1, -2, -3, etc.
You can post some specific plays with your index numbers if you want to and I will go over them with you. You can refer to a page in Professional Blackjack. I have a copy.
