Oh dear, this is the conflict that I am talking about. If Nikki is going to have a fit when you make a fuss of Abbey then she is confusing herself and Abbey as to who is higher in rank. For this to work she simply must accept that as the lower ranking animal to you she must wait for her turn calmly.
Before trying the off lead work you must teach Nikki to sit and wait while you fuss other dogs.
You will need the help of another person or a good strong post or tree to hold her. Tell her to sit and stay, walk towards Abbey, give her a quick pat and if Nikki remains calm go back to her and reward her. If she reacts badly turn towards with your arms crossed and walk look up at the sky until she is VERY calm. Then calmly reward her and try again. Move onto the off lead work only when you are getting no reaction from Nikki even when you make a big fuss of Abbey. When Nikki stays calm make sure you make a very big fuss of her but do it calmly.