Let's try it this way. How do you want to install Win10
upgrade your win7
replace your win7
dual boot with win7
If you're going to replace win7, backup your data and the launch an elevated Command prompt
enter the following commands (*** are comments not part of the command set)
sel vol 1
*** this should be your Win7 volume - the label helps identify it.
*** if the Win7 volume is a different number - select that number
*** the disk with Win7 on it should be the selected disk (there's an asterisk on the left of the selected object)
*** if you are certain that you have the correct disk selected - be very certain! - issue the next command to wipe the disk
exit *** exit diskpart
exit *** exit cmd
Now install Win10 (custom) onto the unallocated space from external media (USB or DVD)
Don't format the disk - let Windows do all the work.
If you want to multi-boot I suggest that you change the disk schema first. I'm not sure that it is required, but you've had some difficulties and it might be that your active partition is also your Win7 Boot partition.
Minitool Partition Wizard makes this relatively easy.
Free download Magic Partition Manager Software, partition magic alternative, free partition magic, partition magic Windows 7 and server partition software - Partition Wizard Online
Download the Free Edition (top of the list)
-- or --
Partition Wizard Bootable CD Free Edition (2nd from the bottom)
I tend to use the bootable CD when working on System disks - just so the OS doesn't argue or complain.
One you have PW ready, I'd like to see a full Disk Managment screen shot before offering other advice.
The process will go something like this
1) shrink C: by 450 MB
2) slide C: to the right so that there is 450 MB free on the left
3) setup the System Reserve volume - I'll need to revisit the specs or look at my system for reference
4) set the new System Reserve as the active partition
5) Run Win7 Startup Repair move the necessary files form C: to System Reserve.
Don't try this at home
Let me know how you want to install Win10, post the full disk management screen shot, and get PW ready.
I'll collect some other information that will help (there's a tutorial on SevenForums that describes moving the active partition from C: or to C: )
This is a bit of work, but it's not too difficult. Most of it is preparation and information gathering. The actual process might take less than 30 minutes.
Then once the disk schema look right to me, you can shrink C; some more (30 GB is enough to test Win10) and install Win10 to that unallocated space.
The key in both scenarios is unallocated space.