The general rule of thumb is - only farm Minotaur 15, and only farm it if you can either run one level 60 + 4 people who need masteries, or five people who need masteries.

So, for you, go with the 800 gems. But be very selective of who you actually use them on. For now, only do your starter.

Thanks.  That is rather annoying scrolls only go to certain champs... I assumed they'd go to a pool anyone could use

The scrolls will go to where ever there are open slots. You can fill up the first 2 rows with white scrolls, it takes 100 per champ. If you are attacking minotaur with multiple champs they will go to any slot that there is room, at random. If you have champs that have a full row of white scrolls, then if you send those champs with your new ones, their slots will be filled and scrolls will go to the new champs with empty slots.

It is not necessary to get all the scrolls for 800 gems, but you can more easily add some effects to most of your champs with the first 2 rows filled.

It all depends what you need in champs to beat the minotaur. Once you get a champ fully mastered (with gems) then that can be used to build up others.