Your idea sounds pretty good, at least from Bitcoin point of view.

Since your customers are not tech people and it's risky to handle the private keys, they can clearly see their funds based on the address, on any block explorer. So you can handle the keys as you think it's best.
Because we talk about others' funds, a solution is 2-of-3 multisig, with key 1 at the owner, key 2 at you and key 3 in your testament. Or, if you are allowed to spend at any time, you can also have a copy of key 3.