I am in favor of this solution if the ProWeapon-only functionality gets vetoed by DavidW. Fireshield spells have always been cool but never as appealing as Stoneskin because of the interrupt-on-hit issue, but not being dispelled by a 5th-level universal countermeasure would make them a bit more attractive. (Speaking as a Sorcerer.) Pierce Shield can still be used to remove all protections in the same manner as Breach has in the past, and its higher level makes stripping every single combat protection more reasonable.
Personally I wouldn't have a problem with Breach not removing specific protections even if the SCS solution is a "hot-swap." At least there would still have to be a choice between dispelling combat or specific protections since both could not be done simultaneously without utilizing a higher-level spell slot.
I sympathize with the desire to improve the threat posed to the party by priests, although they are really meant to shine as a support class, and the observation that clerics work better in a group is actually quite appropriate. I really think the casting times for priest and druid spells need some help, though. Heal takes a full round, for instance, making it an very unreliable combat-cast.
Typically wizard spells have a casting time proportional to the spell level, while lower-level cleric and druid spells often have casting times of 5 or more. Are the priest spells that much more effective that they warrant being so prone to interruption? Another option is to reduce the casting time of Sanctuary and allow priests to use that to heal and buff themselves before resuming battle.
Contingency, though, conceptually represents some mastery over meta-magic, which I don't see priests possessing. (Only a personal opinion.) I would be reluctant to grant it to priests, let alone the AI-revising effort that would be required.