This, of course, is a GOOD problem, as you have both more income and equity than when you started.

I want to help him solve this problem but I don't want to inherit his problem as well.  

The problem, as the Japanese can attest to, is that deflation is excruciatingly difficult to get rid of, once it has become entrenched.

This isn't really a problem as you can delegate these tasks keeping half a watchful eye and only stepping in when you notice things abnormal (with your experience you can spot it quickly), and learn a related subject.The problem is too many diversions to your attention, spreading yourself too thin, and jumping to something new and interesting before really mastering the last.

Calculating the principal is not the problem (as you can see from the worksheet).

That leaves you with the same problem as before, which is that won't adhere to linoleum and could break down under the backer. 

Actually, the fact that you don't have any plans yet is a BIG part of the problem, as it sounds like you've already committed to buying the property.If you don't yet have rehab numbers, how can you possibly know whether it would make a good flip and/or rental?

That has been my problem as well.

Shouldn't be a problem as long as you don't expect to lounge on the beach soaking up the sun.

This would not completely solve the problem as its the original floor boards at 3/4" or so, so I would have to shim the left side also a bit to buy the complete 1-3/8" level.