In my experience, the IRS does not know what to do with 1099-Ks.  Their computers do have some sort of algorithm to match them and generate CP notices.  The problem with 1099-K is that the numbers are often meaningless and include things like principal (as you pointed out), sales tax, tips, rental income, capital asset sales, etc.  Things that are actual revenue might be reported on Sch 1, B, C, D, E, F, 4797, etc.

My approach would be to attach a free-form statement to the return explaining the situation and possibly reconciling the amounts reported on the return to the 1099-K total.  Yours might be easy:

Interest received reported on Sch C $10,000
Principal payments received on Loans $5,000

Total per Form 1099-K from Venmo (etc.) $15,000

Other folks will tell you to run it in and out of Sch C.  To each his/her own (but this approach doesn't work for me in VA since then the gross receipts will get reported on a VA return and a VA locality will be expecting the taxpayer to pay a gross receipts tax with their business license renewal.)

Rick