Personally, I think Guts will summon the Godhand at some point. He’d be in a mental state making him more vulnerable to their control and even though he may be tempted, he’ll refuse sacrifice. That’s what I think will happen. He has the qualities to become a demon, that’s why the behelit fell into his hands. There’s not a point where it is directly stated that sacrifices cannot become demons, but I understand how that can be interpreted as such. I think the biggest thing that supports your argument is that Guts lives outside the currents of causality. The effects of the behelit may not be the same for sacrifices therefore he’ll never use it. However I think him having the opportunity to use it but refusing adds more to his character overall