@KibblesTasty Thanks for chiming in!
Would you be able to clarify two of the issues for me please?
First, what exactly is the issue with being able to cast Hallow via Divine Intervention?
The issue comes down to a lack of clarity around the Divine Intervention and the Magic Action.
Hallow has a 24 hour casting time, and is definitely not intended to be cast as an action. It can do wild things like give all creatures of a creature type of your choice Vulnerability to damage type you choose (with no save!) and that's just one of its many effects. This is normally limited by that you cannot cast it combat, obviously.
The issue is with Divine Invention, it says "As a Magic action, choose any Cleric spell of level 5 or lower that doesn't require a Reaction to cast"; this obviously would include Hallow, which takes 24 hours to cast, not a reaction.
The Magic action itself says "If you cast a spell that has a casting time of 1 minute or longer you must take the Magic action on each turn of that", you have to keep taking the action to complete it, but the Divine Invention spell continues "As part of that same action, you cast that spell without expending a spell slot or needing Material components"
I think that it doesn't work, but if doesn't work there are some other oddities. Like Pact of the Chain says "You learn the Find Familiar spell and can cast it as a Magic action..." but being able to 'cast it as a Magic action' doesn't do anything if that wording doesn't mean "as an action" and means "as the normal casting of a spell". Ultimately its just confusing wording, because anyone that just reads Divine Intervention or Pact of the Chain will
definitely think those are letting you cast it as an action, because calling something that's not an action an action is confusing!
Second, how does the math break down when combining Scorching Ray with Conjure Minor Elementals?
It's just a matter of scaling. Conjure Minor Elementals for some reason scales by 2d8 per spell level when upcast, and applies with each attack you make (including spell attacks). Scorching Ray scales to make more attacks. So, imagine a basic combo of those spells is 3 × (2d6 + 2d8), but up casting them both by 1 level is now 4 × (2d6 + 4d8). That's a very exponential scaling.
The main reason its not a bigger problem is that Conjure Minor Elementals is already a high level spell, but its worth pointing out because its a lot more damage than anything else that existed previously (getting up over 900 damage by tier 4 if I remember correctly without redoing the math, though obviously they wouldn't all hit).
You can combo it with a lot of things, not just scorching ray, that's just the most effective one.