You can sign in with an email and you can share your progress between more than 1 device. Just be sure that you choose USED cloud on the new device to move your save across. If you choose the wrong one you may overwrite your cloud save with a new account
If you do the latter then you will need to go back on the old device and choose use local to update your cloud save with your progressed saved.
If you want to hack keys you need at least 1 key.
When you have 1 key the value to search for is -2 (negative 2)
Now switch to another player with 0 keys and the value is -1 (negative 1)
if your player has 2 keys the value to search for is -3.. and so forth.
Keep on switch back and forward between those players. Till you get down to 2-4 values (cant remember)
Put in -10 (this will give you 9 keys)
You will now need to either open a blackbox or lockdown the values then enter and exit a single player game to lock in your keys.
Once you have done that you can also hack the augment gems. Look carefully at the HEX values near the values for keys. SInce you have 0 augments the value should be -1 (simply change several -1 to -255) and you will have 254 augments if you pick the right value. Be sure to augment once to lock this value.
A word of warning the higher your keys the lower your chances of dropping a Blackbox. (yes its rigged that way.)
So its best to have lots of keys in a low level player, and keep your main characters with 0 keys. If you do get a blackbox you can easily find the values to give yourself enough key to open those boxes. It is easy to search for key value of say 2533 keys (-2534) then searching for -2.
The -1 rule also applies to turrets, money and grenades..
Blackboxes themselves cant be hacked as the contents is already determined when you get the drop. So they are just an piece of gear you cant equip or sell till you open the box and not a count of say 6 items you have got.
You could use something like freedom to buy blackboxes but you risk getting your google account been banned from the playstore. I personally avoid programs that make fake store purchases via online connections.
Its highly recommended to have titanuiumbackup SAS4 so you can have a unliminted goes at finding the key value, when you open boxes in order to narrow down the key value. I would start by hacking money and turrets first thats easier to find.