1. If you check metanalysis of minoxidil studies only 40% of participants regrow hair (SULT1A main theory). If we consider microneedling a "boost" of WNT upregulation that minoxidil already does, it is to be expected some people still don't respond. Also all studies on microneedling as monotherapy have shown almost no results, so its clear minoxidil is required for the "magic". Id say it turns an hyper-responder into giga-responder. A normal responder into a great responder, and so on.
2. I haven't got hyperpigmentation despite having it in other parts of my body. Excessive scarring could cause hyperpigmentation so make sure u arent going too ham. Funnily enough microneedling is being used and studied to treat hyperpigmentation of the face skin like melasma on upper lips with great results.. lol.
And if by hyperpigmentation you mean expanding moles or dark spots you should visit your derm asap for a checkup. It could be skin melanoma (cancer) and needling over that and generating growth factors is definetely a terrible idea.