Moving wet appliances away from sink around kitchen L bend and past oven
Why do the contractors you've spoken with believe there will be drainage issues, and why are you unsure about what they are telling you? They have a strong incentive to do what you ask: it's what they get paid for. If some of them are apprehensive, and unless they are trying to sell you a different solution that is even more complicated (ie. expensive), what else could be motivating their response besides "doing the right thing" ? 

Put another way, the ones that are willing to do whatever you ask might just be willing to move your sink into the ceiling and install your toilet on the back patio, as long as they get paid for it. The ones that are apprehensive get nothing out of it by telling you to not proceed, besides maintaining their professional integrity. 

I would weight heavily the advice from the professionals that are willing to forgo a quick buck to maintain their reputation.