No support. This would mean you are getting just as many ww from the tokens as from the main way you are meant to get them. This also would double the influx of new wolves. A succesful ww encounter is a special thing imo, and I think that would be cheapened if every 10 fails you auto get a new wolf.
I like 50 because it feels like a nice consolation prize for someone who's had bad luck and been at it for a week with little success, but isn't more than a little extra bonus to someone with average or good luck.
Lastly, a pebble tax is not horrible to me. If someone feels it's too steep, they can choose to just miss out on the extra ww, and get the successful campaign encounter ww for their grind, but for people who want to utilize it, it gives use to their pebbles. When pebbles are taken out of the system, it keeps them a bit more rare, and therefore the ones you find on the site are worth more. Right now there are very few ways to use your pebbles, and the most expensive uses are generally the recipes which you only have to buy once. I think it could be a good additional currency sink.
With sake's idea, I think that could be hard to implement, or not be intuitive to players possibly. Instead of that my suggestion would be that on some rollover schedule, it could take more kucklebones for each ww you want to catch in succesion with a smaller initial kb price tag, say it starts at 40, then 45, 50, 55 and so on, or something of the sort. then once the week or whichever it is, it can rollover back to the smallest price.