Bael is exceptionally powerful, but yes, any mortal should be able to use darkness, even without Pyramid Tech. Eramis is an example, as after being frozen, she proved capable of using Stasis without the Pyramid Tech, though that might be due to Witness Interference. Zavala and Osiris can both use Stasis and Strand respectively, as can Elsie Bray, but all of them might be exceptions too. Zavala and Osiris were both previously Lightbearers, and Elsie Bray has a unique connection to the traveler which hurtled her through time upon the failure of each timeline. There are also cases of Guardians using light in spite of their lack of a Ghost. This being relevant in the form of Shin Malphur who is technically a mortal man thanks to the Sacrifice of his Ghost when he was but a babe. And he hasn’t died since meaning Jaren Wards ghost hasn’t resurrected him, which we don’t even know if a Ghost could resurrect someone besides their chosen Lightbearer. Otherwise Ghosts like Blue and Fynch would abandon their respective Lightbearer (Excluding the events of Renegades).

Well seeing as darkness exists in all living things (like the light) I'd say yes but it would vary, for one to wield darkness one must learn from another how to (or as a group)

Elsie is a normal human, well a normal exo She wasn't a guardian and she learned to use Stasis on her own I assume anyone who accepts and communes with a splinter, and accepts the darkness inside them