Hi, you may well be lucky in that it may not have pierced the inner wall of the tyre itself. Tyre walls on the treaded area are about 4mm (1/8 - 3/16") thick and you look as if you have plenty of tread there.
I'd have thought the soapy water test would have shown it bubbling up if it had punctured through. Could just be creating a good seal around the shank of the nail of course.
I've had similar in times past (not rft's though) and i've always removed the nail / tack and monitored the tyre pressure for a few weeks. The rubber just goes back to its original shape. If it has punctured, the likelihood is a very small leak - 'slow' puncture.
You are right to be cautious though and a trip to the tyre dealer is the best option. They will probably remove the nail and can put it into a tank of water to monitor the tyre for a leak.