History isn't stored on the server so it can't be automatically populated on a new device. That is a feature. The alternative, storing the messages on the server or having the means for one device to clone all of its messages to another device, would be insecure.
A 30 character long password is required in order to have enough bits of entropy so that the backed up messages are actually secure.
Grandma isn't moving her data to a new PC without assistance, the person that is assisting her should be competent enough to operate Signal.