Don't worry, so far it appears that the Treasury Dept is on top of this. I'm sure a dead person will be receiving your client's $1200 payment shortly.
if filing a 2018 return, use the address where client is currently living. if the irs sends any notices I would assume you would like your clients to receive them(!). I don't see why they can't use direct deposit for any 2018 refund as long as (again) the account is open now! I have a client who used direct deposit for 2018 refund and another account for his 2019 refund (e-filed & accepted march 30). he tells me the stimulus payment (per irs.gov) went into his 2018 account -- fyi. so yes @IRonMaN irs is really careful with the stimulus $$$