HI Sam,
Thanks for your detailed response. Yes I forgot the image but you nailed it, that was the exact issue! I did not realize I could subtract one spline from the other inside of the extrude to fix it. I tried flat mapping but it turns out that was only part of the solution. SOLVED
I also found that if the graphic was properly centred I could also use the ‘fit to object command’ though this only applied in some cases (ie not the two-edge cases you referenced)
I also learned a valuable lesson – geometry from splines are a nightmare if you intend to project the original texture the spline was vectorised from, as their will always be issues with the alpha if you also want post depth of field – won’t be making this mistake again, will use a different approach. Sadly this time I was days in to the project before I realised my issue so had no choice but to keep going – thanks for saving me!