How would I integrate 80+ wesites customers profiles into a single
database/login portal (which then creates one login for all sites)
where the only unique identifier is email address? Users may have
multiple accounts across all sites but may have changed their details
several times in the past. I cannot ask users if this is their details
due to confidentiality as it may be incorrect. Is there some sort of
varification method I can use?

I have a suggestion that may allow you to solve the problem but in a
different manner than what your requirements specify:

If you are not opposed to a solution that requires your users to
install software there is an option that may meet your needs: RoboForm
(and no I am not affiliated with this product or company; I have used
the product and think it is strong "password management" tool that
could be an adequate solution that accomplishes the same end result as
the central storage mentioned in your question.)

One advantage of this approach is that the passwords are stored on the
client's system in encrypted form thereby freeing you from having to
ensure that data can not be stolen and decrypted through a sql
injection vector.

RoboForm can create strong passwords that end users need not memorize.


More info:
http://www.roboform.com/enterprise.html
Fujitsu example (from url above): 
Fujitsu has several hundred support operators that need to login to 5
internal web sites every time they come to work. Fujitsu installed
RoboForm on every operator's workstation and implemented automated
procedure that executes Login on 5 passcards for these web sites at
the start of the day. RoboForm toolbar is hidden, so operators do not
even know how login happens and they cannot see passwords used, due to
Dual Password feature.

Pay As You Go: Flat Per Seat Pricing. 
Since RoboForm does not require an expensive implementation phase, you
can start using it by simply downloading a Free 30-day Trial of
RoboForm.
Then you can buy any number of RoboForm Pro licenses if you like it,
you can even buy just one license. We also provide Volume Discounts
and Site Licenses.

Customization and Private-Labeling. 
We can create a Custom Private-Labeled or Co-Branded version of
RoboForm if you buy a big Site License