I may look at stocking male LMB. How is it possible to source them with high degree of certainty? (Near 100%)
Why? Are you HBG stunting? Did you read my posts above about the benefit of HBG? If Rex stands by their source/genetics, you should have 97% male - should not be posing a population management issue. HBG are a great companion to SMB/YP ponds, but Cody/etc. have suggested the benefit of refreshing genetics periodically with true F1 HBG to help prevent reverting to GSF genetics over the generations.
I would only purchase LMB from a hatchery selling single sex fish - either that or be very experienced and catch them on their bed and verify with milt the fish you're targeting. In your scenario I do not believe LMB are even remotely necessary - I still don't understand the nature of the management issue the HBG are posing.