I think that you might be over analyzing things but I am sure that I can help one way or another
Grading figures is a personal choice - but to be honest, unless it is something very rare then really all you are doing is paying for someone's opinion - which will be no much better than your own. I know some people like the say they look in the cases and the cases with rails that hold them are only available from AFA or UKG. There have been plenty of instances where those rails have rubbed against the figure and caused additional markings and once they are sealed then there isn't much you can do about this.
As for your situation then if you are just storing them, the clamshells will work just fine - most will go in the standard version (20GBP posted for 50) but a few will need the wider one. Alternatively you can just store them in acid free bags (3GBP for 100) - either will keep them safe and not cause any damage (although it is sensible to open the bags or the clamshells now and again to let the figures breath)
Once you are ready for display then either single figure cases (you can glue the stand to the base) do look very nice and are much cheaper than grading the figure. Alternatively you can purchase some display cases from Ikea or like likes (Detolf and Billy are popular) and display the figures in there. We also so do steps so you can display more in a smaller space
Hope that helps a bit