So if you don't get your J1 because you have indicated immigrant intent, what are you going to do?
Have you even asked them if they can hold off the offer for a month? I know that if I were employing somebody I would feel much more comfortable if they were a permanent resident than on a temporary visa and I would be happy to wait another month. And an employer that wouldn't wait a month for something as important as this may not be someone you actually want to work for to be honest... as you clearly will be in some form of educated job, not desperate on minimum wage.
And if I had left my home country for a job I would also be happier being on a permanent resident basis than a non-immigrant visa, because anything can happen with jobs.
But if you honestly feel your job is more important, I guess you can go for your J1 interview, tell them you do not want to pursue a green card anymore and that they can cancel your DS260, and hope like hell that will be enough for them to give you a non-immigrant visa.
Personally I think you are being incredibly short sighted... but that's just my opinion.