Getting a Keystone of Trials, do the whole rift have to played on T1+ or only killing the Rift Guardian?

I am trying to obtain a Keystone of Trials to test the new greater rifts that was recently added to Diablo 3 on my Xbox One through the 2.1 patch.

Do I have to play the whole rift on T1 (or higher) or is it enough that the game is on T1 (or higher) difficulty when I kill the rift guardian?

Meaning, could I do this:

• Start the gaming session on Master difficulty
• Lower it to hard, and start a normal rift
• Play the normal rift up to about 98-99% completion, then switch to T1
• Kill the last minions and then the rift guardian

And if that works, can I even do this:

• Start my gaming session on T2
• Lower it to T1, and start a normal rift
• Play the rift up to and including almost killing the guarding, getting him close to death
• Set the game up to T3, and kill him, to increase the chance of him giving me the keystone?

I'm playing alone right now so I can easily pause the game to change the difficulty.

I'm assuming that plowing through the minions on a lower difficulty is going to be faster than doing them on a higher one, but will this remove my chance of getting the keystone?