Should I book segments even if full trip is not posted?

Hello! I am currently booking a trip with my airline miles and have three questions, my itinerary is

Sapporo, Japan to Bangkok, Thailand on February 2/13

Phnom Penh, Cambodia to Seoul, South Korea on 3/21

then Seoul to Sapporo on 3/27?

The first two segments are fine, but the third one keeps coming up as no available flights. This is because the flight from Seoul to Sapporo is a new flight on Asiana airlines (my flight from Phnom Penh to Seoul is on Asiana) and its not even listed on their website. It is listed on Sapporo New Chitose airports wikipedia page as starting March 14th, however I am not completely sure it is happening.

So my questions would be:

1. Anywhere I can find out whether the Seoul -> Sapporo flight is actually happening? And if it is, would I even be able to buy a ticket with airline miles only a week after the flight started?

2. Should I buy the two earlier segments without the third? Or if I book it all together is the price/airline miles different?

3. Whats the difference with regards to saver/standard awards if you need to cancel the flight? Will I be able to get the miles/money back?

Thanks so much! And if you have any travel tips in Thailand/Laos/Cambodia let me know :)