I don't think this is really an issue, as a practical matter.  Interest is computed daily, down to the cent (or, worst case, to hundredths of cents), how much the interest is....

UPDATEs can indeed cause page splits, in particular if lengthening the value of a varchar column, which is quite common.  Often the resulting potential page split issues get overlooked in that...

Yes.  Set a control value using sp_set_session_context; you can test it inside the trigger using SESSION_CONTEXT.

As an aside, I suggest you use >= and < for all date/time/datetime comparisons rather than between.  That also gets rid of the need for the "tricky" use of -1...

SET IDENTITY_INSERT [Marketo].[EmailActivity_Temp] ON;
INSERT INTO [Marketo].[EmailActivity_Temp] (
  [id], [leadId], [activityTypeId], [primaryAttributeValueId], [activityDate],
  [DateAdded], [ProcessedInd], [primaryAttributeValue], [CampaignID], [EmailActivityKey],
  [NextPageToken], [LinkClicked], [ProgramName], [Attribute], [AssetName]
 ...

Erwin is a great modeling tool, the best I've used, although it wasn't cheap the last time I was able to use it.

Assuming you have multiple CountyDistrict values, and the table isn't clustered already, try clustering the tables:

CREATE CLUSTERED INDEX DistrictTeachers__CL ON dbo.DistrictTeachers ( CountyDistrict, TeacherId ) WITH ( FILLFACTOR...

For credit card info specifically, you'll need to make sure you follow relevant laws on handling that data.  Just SQL-based encryption methods is not enough, as I understand it, but...

I think the q/problem is:
given a date, if that date is before/on the first Monday of the current month, use the current month's first Monday,
else, use the first...

Not necessarily.  It depends at least on:

Minimum disk read would be 8K, the size of a single SQL page.  SQL does indeed organize 8 8K pages into 64K extents, but I don't believe SQL has to...

Most often you can use CROSS APPLY to assign an alias name to a calculated value, which you can then use in the main query or in another CROSS APPLY,...