Is Primal Strike considered a default weapon attack? In other words will it proc reaping strike and necrotic edge from the necromancer?
No, it’s not. Only Cadence, Fire Strike, Savagery and Righteous Fervor are default weapon attack replacers. There are also a couple more, one from a component and one from a relic.
Yup, as medea says. Basically, if it’s a default attack replacer it will note it as such somewhere in the skills text.
Tho it appears that Crate has modified the text on such skills - kinda doesn’t look so clear about it anymore to me:

Seemingly 'Default attack replacer" skills are now instead going for the line “When used as your default weapon attack” to denote that they are replacer skills. Just noticed this. Kinda odd and lowkey.
So been reading more on this and correct me if I’m wrong but since there is a cool down on primal strike any attacks in between the “strike” would be default plain attacks by weapon.? Would it then activate the passives from necro “off default attacks”? As with many skills in this game they get hidden with way to many on screen effects that I can’t tell if something is working or not. Really wish they would put some sort of character animation window to show a skill in action (many many devotion skills are used without me even knowing…here is looking at my purifier and it’s rainbow of effects procing constantly.)
Yes and yes. You can use many cooldown skills in this manner and I often do this with builds.