You would think that your method should work every time if the seed is viable. That's all you do in nature, just plant the seed and watch it grow.I’ve never done anything but direct germ in soil. I know there are tons of tried and true methods but my routine works for me. Soak the pot to runoff. Drop beans little less than first knuckle deep. Spray top of soil to keep damp. I continue to just use the spray bottle for about the first week. I’ve had great results doing it this way but it definitely takes longer to get sprouts. But, I also feel like it weeds out any weak plants.
The one that stayed small was kept in a 3" pot till she was transplanted and topped right at the same time as she showed sex. The other was transplanted at the same time but was developmentally behind by a few days. Goodluck! Double Grape tends to be wider than she is tall from what I've seen. I had 7 in my 2x2 for a bit and that was intense even with super heavy training and repeated toppings. I ended up getting about an ounce from each but I had a lot of issues. I specifically chose strains that were supposed to stay small, were trained heavily and topped a bunch, and it was still tight.