Both the busses you've mentioned require reservation; the latter bus is operated by Hokuriku Railroad.
If you can hold a Japan Rail Pass, the Pass covers fares and charges between Takayama and Kyoto / Takayama and Kanazawa / Kanazawa and Kyoto.
http://www.japanrailpass.net/eng/en002.htmlI suppose it can be a hectic trip to come into and go out Shirakawa-go in 3 hours or so; it's better if you can reach Shirakawa-go earlier or lodge there.
- Shirakawa Village: Tourist info
http://www.shirakawa-go.org/english/index.html
- japan-guide.com: Shirakawa-go and Gokayama
http://www.japan-guide.com/e/e5950.html
To visit Kanazawa, I guess your plan will do. Thunderbird #36 leaves from Kanazawa at 16:03 ; maybe it' better to stay there longer.
- Kanazawa City Tourism Association
http://www.kanazawa-tourism.com/
- japan-guide.com: Kenrokuen Garden
http://www.japan-guide.com/e/e4200.html
- japan-guide.com: Kanazawa Castle
http://www.japan-guide.com/e/e4202.html
When you lodge in Shirakawa-go and skip Kanazawa....
Takayama 12:50 ;
| Nohi Bus (requiring reservation) #;
around 13:45 Shirakawa-go: tour and lodge;
Shirakawa-go 06:53 ;
| Nohi Bus (not requiring reservation) #;
around 08:00 Takayama 09:37 ;
| Ltd. Express (WideView) Hida #6;
12:01 Nagoya 12:22 ;
| Shinkansen HIKARI #509;
13:15 Kyoto.
# The round-trip bus fare is 4,300 yen per adult.
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