I saw all the stuff about orc druids and that made me wonder something: can a orc / half-orc worship a druidic deity and get powers from her, yet still worship Gruumsh as her main deity and be considered by him as one of his followers?

No.

Your patron is whatever deity is giving you powers. If you have another god in your deity field, but pay your main worship to another, expect to lose your divine powers.