5D

Do you think Gray can, and should, trust Seichan?

to a degree. they work well together but he has to remember that she has also double-crossed him multiple times

To a certain extent, I think he can, more so than Nasser at least. Seichan has cooperated with Gray in the past; plus, I think she likes him (though she'd never admit it!).
I'd always be wary of her though, you never know when she's going to turn - she will always have her own agenda, though I don't think she'd ever get to point where she'd kill Gray.

Their history together proves she can't be trusted but since they are working towards a common goal it might work out in this case.

About as far as he can see her. She needs him and she's working with him now but that's going to soon be over.

I think he can trust her with the common goal but she could turn against him pretty quick.