I have 150,000 in United Mlieage. I am looking to book a roundtrip UAL codeshare operated by ANA flying from JFK to Tokyo and below are the redemption rates on United website

55,000 mile for one way economic flight ($1,500.00 round trip if I chose the changeable option)
170,000 for one way premium economic ($2,400 to $2700 round trip depending on the arrival time)
200,000 for one way business class ( $10,000 round trip)

I haven't redeem for international flight or codeshare flight so I have a lot of questions that I hope someone can help me

1)If I don't have enough points, can I pay in part points and part cash? I think i read that ANA has to to be redeemed fully in points and must be round trip.

2) I still have points in my Chase Rewards that I can transfer to United Miles. Is it worth it for redeem on a business class for 200,000 miles? Or should I buy a upgradable economic flight. Base on my research it is hard to bid on ANA upgrade. They rather leave the section empty than sell the premium seat cheap. Does that apply if I am booking via UA codeshare flight. Is there a high chance I can upgrade to premium economic via United miles

It seems that the redemption rate for premium economic his too highs and look like a better deal if pay with money. With the amount of points (170,000, it seem like a $3400 worth of cash value to redeem the premium economic round trip). And it seems expensive if I buy a one way ticket and redeem the other leg.

I look at the chase portal buy they don't offer the premium economic class. I was thinking of using my chase points to book a ticket.

Thanks in advance for your help. Any advice is greatly appreciated.