If you are swear by W7 then stick with W7 and forget W10 for good
For the record, even if Winblowz10 is free after the deadline (7-26 I think), I would NOT even dare taking a look at it
Too many problems with this fresh new system, thanks to Microsuck's effort to exploit the public as guinea pigs to test for bugs etc.
Most ppl opt for the free upgrade becuz they have valid W7/8.x keys and can retrieve the generic W10 key, then roll back to whatever older system installed
That however does not work well with W7 since restoring from W10 to W7 will result in a very unstable W7 which requires complete reinstallation
That said, I think your method of backing up the W7 system might work, just secure this W7 system backup becuz it will mean everything to your PC