If you don’t have a warlock in your group, you can still get a free healthstone for the start of any mythic plus. You just need to queue for proving grounds in order to get your healthstone. Once you enter, grab your free healthstone and leave instance.
You don’t need to queue via your garrison. In every order hall in Broken Isles, there is an NPC that allows you to queue for proving grounds.
The best part is you do not need to leave group in order to enter your proving grounds, you can do it while in your mythic plus party. But you cannot enter if you are currently in the LFG queue looking to fill your group, so you will need to either do it before, or after the group fills.
You will only get 3 charges, so once you have used them, that is it for the dungeon.