Help with Jaffar

I would use his basekit first and see how well he does.

Imo wasting several resources to make him viable won't be worth it if he'll perform underwhelming

I think it's pretty useful, refine his dagger and give him deathblow+windsweep+savage blow and poison strike seal so you can chip everything. He can't double most mages anyways even with a speed boon and LnD. Pair him with a dancer and destroy whole teams in AA

He really needs investment.
Luckly you at least got the greatest boon alongside that bad bane.

This is what I'd do