Ok, so here are 2 links to the place in Rebun, i can really recommend it, also its far from fancy and dorm room style.
link 1 and
link 2 The phone number is on the HP, don´t forget to add the country code. I found a short video from someone who stayed there, see here
YouTube - Hokkaido Cycling Tour 2009 - Part 3/4
I think the B&B in Furano looks kinda nice. (the one in your link)
here is a little bit cheaper place (6000yen for 2 person), its about 10 min walk from the station. More info to Furano
here
The price for the mount view hotel is just to high and it looks like a huge resort hotel... i prefer small traditional places, ill have a look if i can find something.
Ther is a YH in SOUNKYO Onsen as well, if you call ahead you might be able to get a private room there paying around 6500yen for both without food. Eventually they can arrange fish/vegetarian food for you or you could cook. They have some english on their
homepage and there is also a phonenumber written down. Note that you can stay here for the same price as a member even if you aren´t one but you cannot rent a private room together (male and female) if you are not married (thats the problem with some youth hostels...)
Like said before, for rebun i definetly recommend the Momoiwaso. The place you choose looks quite huge as well and surely isn´t as cosy as a smaller place.
I was also wondering if you really have to stay at Sonkyou Onsen. Why do you want to go there? Because it is an Onsen or because it is in the national park? I have only been to Asahidake wich si around 1 h by bus from Asahikawa. That is a small collection of Hotels and there is a ropeway to the top where you can do some hiking. You can see the pictures in
this post at the end
hope this helped a bit. if you have more question feel free to write again.