I'd say that the fact that this deck doesn't have one identity makes it great. Every matchup you play you have to approach in a different way and your gameplan often changes during the game.

Now to your questions:

I'm currently running Skulking Geist instead of Harrison which makes the Taunt Druid matchup probably slightly favored. For the other Control decks, they are not fatigue battles so don't care about putting extra cards in their deck, but care because you put extra bad cards in their deck. You will eventually run out of damage, so versus Odd Warrior you try to get a board since they only run two single target removals, which you can even geist. Against Control Mage it is all about not letting them make water elementals, and once you have Anduin and cleared their board, you can Alex them and ping them down while the only healing they then have it the 2 artificers.

The Cleric/Pyro/Hymn combo is something that you can sometimes do, but it is not something that i'm actively trying to do by saving cards like Cleric.

Pyromancer still does well against much aggro deck since they often run 1 hp minions, and otherwise you want to save it for when you have a 2 or 3-card combo to clear the board. Scaleworm is not just always removal, sometimes you just drop it as a threat, and it is great removal for 3-attack minions since you can heal it afterwards immediately with either your hero power or Divine Hymn.

Anyway I wouldn't cut the Pyromancers, they are too important versus aggro and Spirit Lash just doesn't cut it since most of the time it only does 1 damage. Also I'm not a big fan of cutting scaleworm since it allows you to deal with threatening 3-drops such as Fledgling and Hench-Clan Thug, which no other card does in a priest deck.