Hi there!
Hmm, wonderful plan!!
1. No, you can't book in advance.
See details from;
http://www.premiumoutlets.co.jp/en/gotemba/access/bus/detail18.html
2. Either way, you have to get Shibazakura Liner from the festival site to Kawaguchiko.
Shibazakura Liner is 2,000 yen for adults.
>by bus
Take highway bus from Kawaguchiko to Matsumoto.
You can book from here;
https://japanbusonline.com/Detail/11500270001/1/4192023001/4202029/
But it seems to accept 3 weeks in advance.
>by train
Take train from Kawaguchiko to Ootsuki station, then change your train to JR to Matsumoto.
There are a few express train called Kaiji from Ootsuki to Kofu, then change another express called Azusa from Kofu to Matsumoto.( departs from the same track)
You can book online from;
https://www.eki-net.com/pc/jreast-shinkansen-reservation/English/wb/common/Menu/Menu.aspx
Also you can book either of expresses at Kawaguchiko station on the day you on board.
I think, train would be working very punctually even though it during Golden week.
But, bus will be the best easiest way to take for you because you don't have to change your buses only even once from kawaguchiko.
Traffic may be gone bad, yes it is expected, but you can take a nap to prepare or cure your tired body during the trip in the bus...
3. Yes, they don't accept booking for that bus in advance.
Just be at the stop and pay to the driver the fee when you on board.
4.Yes you can from;
https://japanbusonline.com/Detail/11900040002/1/4162019/4212032/20170429
But it starts to accept 1 month advance on the you'll on board, so please try on 29 March.
5. I searched traffic congestion prediction on the day you'll travel.
It seems not so bad though it will be shown up suddenly because of some accidents or for other reasons.
It predicts approx 5 - 10 kms traffic jam might be occur somewhere between Kanazawa to Nagoya.
It takes approx 30 mins to get through that jam, to Japanese it's not a big deal.
But if you travel punctually, I recommend to take Super Express Shirasagi, or the combination with Shirasagi and reserved seats of Shinkansen.
Some Shirasagi will take you directly from Kanazawa to Nagoya.
But it's not, you'll need change your train mostly at Maibara station to Shinkansen to Nagoya.
Bus will take approx 4 hours(+ jam or - good traffic).
Train will take approx 3 hours.
Hope this can be your help!
