If I start an amend online but never send it in because I changed my mind, will it affect the processing of my return?

No. Amended returns have to be mailed, they cannot be e-filed. If you have not mailed it the IRS has not received it. It will not impact your original return in any way. You can unamend your return by carefully following the instructions in this FAQ: https://ttlc.intuit.com/replies/4170678

If you changed your mind, then you need to "undo" your amendment. Otherwise it's perfectly possible (and highly likely) that the information you import from your 2017 return into your 2018 return next year, will be wrong.