What I’d do is leave it on overnight. That should let it finish indexing. And just to be clear, did you migrate from a previous Mac or do a clean installation? If you migrated, there is a chance some errant app is running in the background. Try a safe boot (boot holding the shift key) if that’s the case. The i7 shouldn’t be “overheating.” My i5 had Windows 10 running in Parallels, Quicken for Windows running in Crossover (a WINE app), and Excel for Mac, and the fans never went on until I started installing Windows updates.
If you do decide to get the i5, what I’d do is order it now, and then wait until Day 14 after your i7 arrived to initiate the return. That will give you an additional 2 weeks to actually receive the i5 before you have to get the i7 back to Apple through 2-day FedEx (meaning you can use the i7 for up to 26 days and minimize the time you are without a MacBook Pro).