In my experience, Mail merge is aimed more at merging in short bits of text, not large formatted sections of text. The formatting for those small bits of text are then part of the main document and the data to be merged in are in a structured (i.e. comma separated or tab separated, etc), "list" document. It is typically used more for things like filling in address blocks of form letters, creating mail labels, etc.
So, you can likely do most of what you want, but as you said, the "fly in ointment" is likely the rider that you want to do. It also depends on what you want to do for the "Economics Section". If you want to have it be an entire section, then I am not sure how to get that done. OTOH, if you just want to fill in things like monthly rent/lease payment and other little pieces of a reformatted section, then that can be done.
What I would probably suggest is that you create different main documents and associated data files for each overall lease agreement (i.e. at a minimum a standard lease and then one with the rider...and others if you have different riders). So that would be one main lease document with an associated data file with all the entries that would use that lease document (i.e. a spreadsheet along the line in the previous post that would columns for property name, property street address, property city, property state, property zip, tenant name, rent/lease amount, etc...i.e. each data item gets its own column and then the rows are the individual lease entries for each lease you want to create/print) and then do the same for each variation on the lease document (i.e. different riders or if you have different sections, etc).
If that does not work, then I don't know of a better way to do it with Mail Merge.
So, beyond that, you might need to look at something like a custom database that either you create or have someone create for you.