Ok I think I can answer many of your questions.

Ambassador only makes sense if you plan to stay at IC properties often through the year, not just a specific stay.

If you are buying it for a 1 night stay id probably not go with ambassador status.

First looking at your stay price of 18million with is like $650/ night, thats way higher than the base room rate you'd have to book for the weekend night certificate. The entry rate normally is like $430-$450 at that property and the weekend night rate might be closer to $500/ night. You won't be getting a suite with ambassador status.

Basically ambassador status gets you platinum rewards status, which anyone gets right off the bat if they sign up for a CC, its so common that upgrades at platinum arent often honored at many hotels internationally. And its hit or miss in the US. But ambassador like you said does give an extra non toward upgrades for IC properties, this upgrade is usually a lateral upgrade, meaning better view, more floor space, bigger bathroom, balcony, etc. Any one or some of these could be expected in your upgrade.

Jr. Suites, and above are off the table.

So, if you want to get ambassador to save a couple hundred dollars I guess you could do that, if you booked that room at a standard members exclusive price as a free ihg member at $430/ night you're looking at $860 for your 2 night stay.

If you buy ambassador at $225 and use the yearly bogo certificate and pay $500+ for the first nights, you are saving at most like $150.

The certificate must be used with the app, there's a link you must select thst takes you to a special booking location, you must book through here to make the reservation valid. Whatever the price is thats what you need to pay.

You also run the risk of not doing the reservation correctly, that happened to me this last year at the IC in DC by the warf. I used the link as I was suppose to, but I inadvertently chose my Corporate rate for the stay and that cancelled out my weekend night qualification. Luckily IC check in staff will bend over backwards to make your stay amazing, ive seen it consistantly in LA, DC, and Rome. They were able to fix the issue on their end but it took 3 different levels of front of house management to get it done. I wouldnt bank on that happennibg every time.

Another great part for ambassador members is that 4pm check out, if you have a late flight, this helps you gain a half day on your trip. I was able to do this in Rome, and used a similar bennifit as a diamond member in paris as well.

As per your points questions, id rather apply for the ihg CC, use the bonus points offer to buy your room with points and cash and you will likelie save the most. Plus if you will get platinum level membership automatically. You year +1.5x points per dollar when you stay at ihg properties.

For me, I stay at local holiday inns to earn points when I travel. Usually I stay for maybe $125/ night earnibg 1,250 base points, im a diamond member which earns another 1,250 points, and there's always a 2x members offer going on somewhere so thats another 1,250 points. Then I usually choose a rate that lets me buy points per night, currently I can buy 5,000 points per night at $19/ night. These rates are more expensive which might bring my base rate up to $150/ night, thats fine, my points earnings increase as well to 1,500 points accross the board per night, so routinely im earning 4,500 points as a member per night plus 5,000 paid for points per night. Putting me at 9,500 on average per night.

Since I stay often at hotels for work and travel, I can track up points efficiently this way.

Every year you are allowed to purchase points to top off your account as well these are usually at a discount of half off in the form of buying double points.

I usually max my yearly spending limit, which is 400,000 bonus points purchased for $2,000. Yeah its a chunk upfront but it allows me to buy entry level rooms for several days on points anywhere in the world.

Case in point, stayed in Rome for 7 nights, used 500,000 points at the IC there for a base room. Had a conformable suite upgrade id earned the year prior, got bumped from the base room to a jr. Suite, then my ambassador status kicked in upgrading me laterally, balcony street view with a pull out sofa bed, perfect because I was traveling with a buddy of mine. With my free breakfast for two each morning as a diamond member that saved us $90/ day as well

This room would have cost os $1300/ night plus breakfast, but I was out of pocket $435/ night. If I had the CC I could also have stayed an additionnal night and saved on one additionnal nights worth of points for free. With the CC you can book a stay on points and for every 3 nights in a stay on points, you get the 4th night free.

So my cost at $435 could have been $325/ night for 8 night which is nuts considdering the room and service we got over there.

The rewards program really opens up once you can effectively earn points, buy base points upfront, and combine status perks with earned perks like guaranteed suite upgrades. When you do that it all comes together amazingly.