I have 2BR Mountain View, but I see January 2020 wide open.
Ko Olina owners have an added trick for availability, especially during hard to reserve months.
Many owners opt to use the lockoff option, so lockoff units go first and fast. However, long after the lockoffs are gone, the Single Entry Villas are still available. These are 2BR units without a lockoff door. I've seen July Full Villas available 10 months out.
If you are able to find one or two MKO OV units for sale and you sign a contract, make sure you have the owner reserve the week you want now and add the confirmation number to the sales contract. Remember that it can take months for the deed to transfer, and by then it may be too late to reserve the week.
EDIT: Just re-read the OP and although you are saying your FIL may want to buy a couple of weeks, you are looking to "book" January 2020 weeks not. Not sure what is meant by "book". If you mean renting from an owner through TUG or Redweek, then there is no rush. Those weeks will be plentiful and easy to rent. If you mean booking through Marriott, then I don't recommend that unless you somehow get deep discounts through Marriott. It's much more expensive than renting.
My response was based on buying the week - entering a sales contract now and having the seller book next week's use for you. That is the most cost-efficient way of doing it in my opinion.
As far as getting a marginal view vs a great view, the room assignments are not done until very close to arrival day. If you own a couple of weeks and give your room request a month out, you will probably get a good view. If you rent and don't let Marriott know preferences until a few days out, you will get a nice room overlooking a dumpster. There is no excellent view guarantee.
So you are not under any false assumptions - when you purchase a deeded week, the deed will in fact list a specific unit. It may be the best view in the whole building. That is only for deed purposes; it means nothing in terms of actual usage. Your use is based only on the view category. Most owners will never spend a night in their deeded unit.
Having two deeded weeks with the same view, and reserving two back to back weeks give you a pretty good priority. Even if you buy from two different sellers, have each reserve one of the Jan weeks, as long as they are connecting weeks. When the deeds finally transfer you will be golden. Marriott won't know or care they were originally reserved by different owners. The confirmations will show you as owner of each week. So timing the purchase simply means lining up the right sellers who are willing the make the reservation.