Francois Vander Meulen
first question. Clan J (blu 0-1) doesn't have to retreat and must stay, because it cannot stay 4 hexes away from any green clan. AND it doesn't suffer any step loss. is that correct?
Yes, it is correct.
1) should this unit retreat, it would go into another enemy ZOC. since it can't move in another EZOC, does it suffers any other step loss? if yes, where in the rules is this stated?
Yes. The defender stays in place and suffer one step loss. It is in 11.7. Quoting the rules: If the value ‘X’ or ‘Y’ contains an ‘R’, then the unit(s) concerned must retreat one hex without entering an EZoC, otherwise it must suffer 1 additional Step Loss
2) after all modificators the result is d21. So defender loses two steps. since there is one defense unit only, is the "1" absorbed" by the same unit (3 step loss), or is it ignored?
it is ignored. The second value is only relevant when there are more than one defender.