My guess is that it depends on how you play your healers and the rest of the team. If you're sticking to the double fortress, then I feel like you may as well get it in with fear because even if they double you, they get walled by 41/47...twice. Can healers use guard? Either way, it's probably easier for your teammates to drive up her speed than to drive up both her defenses (which is what fear+ indirectly does). The thing is that your offensive teammates probably prefer her to use slow+, not for herself but for them. But if most of the team is stacking drive spd, you might get her to 40 effective (count the 7 debuff as added to her speed instead of subtracted from the enemy's)...45 isn't easy to hit, but it's not terribly difficult, basically means you lose to hyper-offensive units (who probably will have the attack to crack your 34/40).
If your team exploits the chip damage support well and isn't afraid of vantage/desperation, pain+ double sb is an option...I feel like there are better units to do this with, and you're probably sacrificing a lot of tanking potential?
40 res seems good for putting a ploy in the C-slot, so it's an idea to cover whichever of atk/spd your staff doesn't debuff.
As for healing staff, I'm not a fan of rehabilitate; if you take a lot of damage and use her to tank a lot then martyr might be your best shot because she's not debuffing whenever she's healing so she will want the extra bulk. But it does mean, like you said, that she's almost obligated to tank, which is probably the case anyways if you're throwing her forward to debuff? To me it feels strange tanking with a support unit. I should try it with Azama sometime.
On a side note, can you inherit the + staves to her? I thought characters need to be 5* to inherit them, and then you would have to merge that 5* onto your 4*