That "pulled the whole thing sideways and made the buckle worse" part sounds real familiar. I had a small den last year with a bay window and the same thing happened, the stretcher kept wanting to walk the carpet toward the open side of the room. What worked for me was setting up so I stretched away from the odd corner first and saved that spot for last, then worked it in with the knee kicker and a stair tool just like you did. It's slow going but you can feel the backing give a little at a time instead of fighting the whole room at once. I still use the power stretcher on the main field of bigger rooms, it's the weird little corners where it stops helping and starts hurting. Takes some patience but the carpet tells you when it's tight.

Does the simulator even matter if the print shop swaps paper or ink on you without saying anything? You tested a PNG on your screen, but the thing that actually wrecked my last batch was a different printer than the one I proofed on. Patterns help sometimes, but slap too many on a small tag and it turns into visual noise that slows everyone down, including the people with normal color vision. I'd rather push the vendor to stick to one fixed palette and sign off on a physical proof than trust a free browser tool to catch everything.