www programming

Yes.
As I mentioned before, I use Amember Pro to handle the registration and it maintains a user database which remains in sync with the phpBB user database. It is also designed to integrate the user database with Wordpress. To make sure this was not interfering with your plugin, I disabled the Wordpress integration yesterday - but it made no difference.

yes look, i'm over just now, for 1.6.9 release, but blocked by this, i want before 1.6.9 ready also for MU, at least with a single phpBB ... hope to resolve right now and that no evil come out in the mean time.
I had test all working fine, but i had not try out the two possibilities: an user can or not create a site.
The code work as on my test if the user not create a site. Than it is correctly added into phpBB in the right way, as deactivated, and activated when confirm.
But if the user create a site also, than the user as the code is, isn't added at same time into phpBB.
So, the problem here, is that we need to find out a way to register the user when create also a site.
The problem is, that the registration flow change a little between with site or not registration, and i found that a specific filter, do not fire (but should!) at when specific event should fire the code. That's strange.
the filter that do not fire is: looking ...

I'm not sure if I'm understanding this but, just to be clear, after I installed and activated WP_w3all I did not create a site. My multisites were created in Wordpress long before I tried to use WP_w3all to integrate them with my already existing phpBB forum.

The order of events was: I created a phpBB forum, then I created a Wordpress multisite with 6 subsites, then I tried to integrate WP + phpBB with WP_w3all.

nope, the above was working fine ... yes maybe i've go with a too much advanced log that lack of good Eng also.
I've take my three hours dive in into WP MU to see how it work ... i had move this along the time, finally

all should work now, but there are limits:

1) the linked phpBB need to be the same into all subsites.
2) if plugin is network activated, the subadmin user need to setup into his site the same config for the integration plugin, because if not, when login the site into a subsite where the plugin is not configured, the phpBB cookie is not released and when he visit main site or another subsite where the plugin run configured, will be logged out.
The same need to be done on all created subsites.
With new code, all on my tests work fine, even what was apparently working before, and was not instead.
Admin and subadmin can add users, even on subsites, all work fine (for what i've test). The login flow is ok, users are added as deactivated and activated into phpBB when confirm signup.

There is a problem about activation/deactivation if the user has been deactivated into phpBB: onlogin, the user is not deactivated into WP: this is a secondary problem, due to the fact by the way, a deactivated phpBB user can not login-

All is ready ... going to put all fixes about iframe redirect for notification (hope you'll experience the click on email after i'll apply all here, in few minutes from this post, so you'll see for first, finally, a correct email link redirect to the forum iframe page ...
many changes to page forum has been done, and several bugs fixed.
Posting 1.6.9 here in short after, and 1.6.9 will be released after a very little test more, if all will go fine ...

and this is what i have into my localhost: i need to rebuild all in mind to write tutorial and not forget any particular.
If in the while you can give a try ... we see if we now is on the right way ...
consider that also page-forum.php is the one modified with latest additions, but maybe before to rebuild, is necessary to have all tutorials about how to use and the new overall_footer.html js code.

i see the last bug about scroll: if i insert a new post, the page scroll to top, and to the last one inserted post after insertion, if the post contain more pages ... i assume can be also secondary, we will fix after ....

OK, I gave it a whirl and this is what I get:

Install via WP network admin and then network activate.

I go to a subsite and get 'The page isn’t redirecting properly' (URL is http://mywebsite.com/subsite/wp-admin/profile.php)