Wi-fi won't start up [Mid2015 15" MBPr, 10.10.5]

During the last 18 hours, I've read an immense amount of threads on Wi-Fi -problems in Yosemite, and while I have not found a single matching symptom description, I've tried every tip (NVRAM, SMC, thrashing prefs, new location, command line, new user ... I can't even remember all tricks) - all to no avail.

On thursday night, I was working with my Wi-Fi as usual - no problems.

Friday noon I opened the machine and now I have the following problem:

When opening from restart, the Wi-Fi shows as on (menu bar), but not connected.

Once I click the icon, it shows as if it could not find any networks - no surprise there: Yosemite has been quirky, Usually a Turn-off/Turn-in cycle would remedy that.

If I now Turn off Wi-Fi it will not turn back on. No feedback, nothing in the console

If I after restart do not click the menu bar icon, but go via system preferences, it shows as off (although the menu bar icon says otherwise), If I click "on", nothing happens (again).

The only situation when I get it to do anything closely to normal is when I thrash network prefs and restart. Then Wi-Fi starts up as normal, shows my network (does not connect automatically as it should), when I select to join it, the Wi-Fi shuts off, and we're back to where we were.

The console does not show anything clear-cut. This is all which the console says after a (thrashed prefs) restart:

Sat Mar 19 09:30:55.127 ***Starting Up***

Sat Mar 19 09:30:55.488 <airportd[56]> airportdProcessDLILEvent: en0 attached (up)

Sat Mar 19 09:32:35.339 <kernel> AWDL Sync Enabled 0

Sat Mar 19 09:32:36.204 <airportd[56]> _handleLinkEvent: WiFi is not powered. Resetting state variables.

Sat Mar 19 09:32:36.205 <kernel> AWDL Sync Enabled 0

The fourth line (Wifi is not powered) is what occurs when I try to connect to the network.

Just to pre-empt some questions: Between Wi-Fi working and not working:

- no updates were installed

- no software was installed

- no kernel panics

- no knocks or drops

What next?

I have recent backups (user profile and other folders), so a reinstall would not be out of the question, but to my surprise I can not use the recovery mode for a reinstall, as I do not have an internet connection (even there) - the symptoms are exactly the same.

While the diagnostic does not show anything amiss with the hardware, I have some doubts - according to my logic a software problem would not persist in recovery mode.

I also have a bootable 10.10 on a USB-stick, but somehow the machine will not start up from that (selected startup disk, but all I get at startup is the "denied" symbol)

In short, I'm stumped - it's rediculous.

All help appreciated.

P.S. notwithstanding the Wi-Fi issue, the machine works - I'm using it even now (bluetooth tethering to iPad for internet access)