My sister's employer has established an HSA to fund the deductibles in the high deductible health plan. I understand the fully insured plan needs an SPD, do either of the plans require a written plan document? Does the HSA require a written trust? One more thing, is the trust subject to 404 ( c ) if it is a trust? thanks so much.

You aren't even close. The employer doesn't open the HSA the participant does. The employer may help but the account belongs to the participant. As far as SPD goes all health plans need one. HSA has nothing to do with the SPD or health plan. It is simply a funding method for future care.