Can't really understand how it would be counter-intuitive, seeing as there are other characters with much worse scaling working, but I'll still answer your questions.
1) 4-set Archaic Petra can be nice, as the additional Geo DMG% and the DMG buff from the 4-piece is great - except it's highly conditional and only beneficial (in consistency) against single element fights. Conditioned players with 4-set Petra probably have learned how to not pick up other elemental crystals, but that can't be expected of everyone.
2) We have tested Harbinger of Dawn. While the DEF does scale the transient blossom damage, it still takes a considerable amount of DEF to be effective. However, I won't lie and admit I am considering doing another full test run with all DEF stats later to see how viable the TB scaling can be.
3) Yes, base multipliers and scaling alone are important -- however there are characters who have much lower multipliers that still work. Take example our most famous example yet: Zhongli. Low normal attack multipliers, Xiangling has better hit combo, and needs a lot more artifact concentration to be "viable" (or whatever people judge as viable in a PvE game). Yet, there are still players running CP Phys DPS Zhongli and enjoying it, while still being helpful -- What I'm trying to get at is viability is a stretched term for a game like Genshin.