I found that the headings from SkyDemon/Wind was different to my "Manual Calculations".. only by +/- 1 to +/- 3 degrees. I still got to Cambridge flying my manual PLOG
Can you maintain a heading to within 3 degrees?
What you did was sensible, prepare both plogs and compare them, any large discrepancies would indicate an error somewhere. As for interpolating winds, remember they are forecasts so by definition are a guess by someone, try not to over analyze the big picture.
Happy flying