Q: error install windows 8.1 on a 2010 macbook pro 13 inc

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Although you managed to "hack" Boot Camp Assistant and you can create a Windows USB installer, you will not be able to boot from the USB drive and you will get a nice "No bootable device" screen after restarting your Mac.

Apple has set up Boot Camp Assistant in order to show the option to create a USB installer only in Macs which support it. In the case of the MacBook Pro, only MacBooks Pro with Retina display support that option, so you are out of luck. You need Windows on a DVD.

Your Mac does not support W8.1. Please see System requirements to install Windows on your Mac via Boot Camp - Apple Support.

You Mac can not use a USB-only installer for Windows. You need to use a DVD and a USB, both.

Do not modify Bootcamp Info.plist, it will cause more problems.

You have an option. Fully install W7 using USB and DVD. Upgrade to W8.1 as W7 is now fully self-contained. You may run into driver issues, but W7 drivers should work for W8.1. You need to test and verify.