So I 'was' thinking e-brake and then you said it broke and you got a kit. Did you install the kit already? If so, I think you've just got to work on your adjustment...which I know is a pitb to do. Easy, but a pain.
If you didn't install it yet, that's my guess also.
On the subframe bushings...I'll say this: my subframe was repaired last year a week before the settlement period ended. They found one cracked spring and I asked them to check the bushings...they'd have been glad to have sold them to me, I know...but they said they were fine.
When I replaced my front sway bar bushings...that reading here made me think they have to have failed...I compared the old (180K miles at least) to the new. Virtually no difference at all. The rubber might have been slightly less resilient, but it was still rubbery.
I think the rear subframe bushings are similarly massive hunks of rubber not likely to fail so badly. Not saying it's a bad idea, per se, just that I think they're probably fine and that based on your sounds, I think e-brake. Squeaking and scrubbing...perfect symptoms of an e-brake in my book.
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Mangofy your E46!