Need advice on adding User controls

I'm adding a couple of usercontrols to ProductVariantsInGrid.ascx. I've copied the pattern used by the existing TextOptionRequired and SimpleTextbox.ascx so that I can enable my new controls Product admin.
This involved making the new controls and changing the datalayer modifying the Insert and Update productvariant stored procs and adding columns to the Nop_Productvariant table.

Now I'm hoping the is no new release of NopCommerce for a long time ;)
So, have I done this the "correct" way or have I missed some simpler plug-in model?

Yes, you've done this the "correct" way. Hope this topic about adding a new field to an entity in nopCommerce will also help you