Yes, you can tap a treasure without sacrificing it
(Plus, it's only a one-of, so cutting it wouldn't really reduce the deck's cost meaningfully anyway.) The one rare that can be cut is Castle Locthwain, which, as we discussed, is very good and, outside of costing some wildcards, is mostly a free-roll in the deck but not all that important
The same rule applies to cards with possible cleaning or alteration: uncertainty should not be passed to the customer through confident language
Aerith can hold her own in some contexts (a sideboard card for some control strategies where the opponent doesn't have a lot of removal, maybe) but she's probably a cog in a lifegain deck
My goal is to gain life like crazy, until I can bring out felidar sovereign or use chalace of life and death to win the game