Well, I think I solved it 
Solved it in exactly the same way I solved the instructions before, so the solution was there all along 
So my working row (the one with Prev/Next button) finds what to load based on template_current_row which in turn is a combination of pattern_id and incr_row. Prev/Next actions are a simple Increment +/-
So, in my Pattern Instructions table, instead of numbering steps with a single number, I write out a number per size (not sure if it makes any sense, but here it is ). Then I slice it, the same way I slice my stitch counts, and load the appropriate step number using the same logic as stitch counts (Single value column with Row set by lookup). I then use this new number in my template column the same way I had it before, but with a single number. So all my rows that need to be skipped, that don’t have instructions for that size now look like pattern_id_-, while rows that need to show up have sequential numbers. 
If the explanation above doesn’t make any sense, here’s the prototype and I can put together a diagram tomorrow.
oh, that’s very interesting… I want to give it a try just to see if there’s anything I can learn from it… I suspect that I might still run into the issue of “skipping rows with no instructions” since Prev/Next button is a simple increment. So yes, if I’m understanding you correctly, it will pull up the correct instruction, but in some cases it will pull up “empty” instructions…