Data Warehouse measures without a corresponding dimension

This is how I would approach it myself. Remember to think of your Facts as "actions" or "verbs" and your dimensions as the the descriptors of your facts.

So your invoice and invoice line items are both facts. One approach to dealing with this is what you mentioned, making the fact table to the granularity of the invoice level line item. This will lead to redundancy of certain things like you mentioned. This if fine for things as an billing address dimension and certain invoice level facts such as a tax could even be broken down to the line level item and it could then aggregate back up to the invoice level amount.

This approach can break down if you have invoice level facts that don't break down nicely. For example, are customers allowed to have more than one payment method? Could there be more than one type of marketing material attached to the invoice? Does the marketing material correlate to specific line items?

Asking these questions can definitely lead you to creating multiple facts such as FactInvoiceLineItem, FactInvoice, FactPayments, etc. that you end up tying together with something like invoice number. Doing this will give you more flexibility to analyze specific facts better.

If doing this approach and you are building cubes off the fact tables, I would recommend using views to load your cubes. This way you could have a high level view for the invoice that joined your various fact tables and had measures on there such as number of line items, number of payments, etc.

All that to say, I believe going that route is much better than splitting out your fact tables that have the same granularity.

Feel free to respond with more details if you need more help hashing it out.

My answer is a big YES - you should store business logic in data warehouse. That is one of the ideas of data warehouses in the first place. If you have tens of reports showing or filtering leads imagine if a rule to qualify someone as lead would change. Also, what if you have to access DWH data with different tools/systems - you'd need to replicate all the logic. Lastly, calculating this kind of labels can be time consuming and that's why you'd like to have everything precalculated in DWH for the reports to run fast.

To answer your first question - yes, you should go with SCD. There are couple apporaches and you should choose the one that suits best your requirements - it's all about how users want to analyze data (this is where the desing should start.