If we are talking about 11 speed cassettes, the Shimano 11-28 skips the 16 gear. Many other wide ranging cassettes miss the 16 gear to. I am saying that I think the 16 is critical for typical cruising power and cadence on the Kona course. Again, in my case I was in that gear for 29 minutes so that is a good chunk of the course. If I didn't have the 16, that means I would have spent 29 minutes in the 15 or the 17, either of which are doable, but just not the perfect gear to be in for those times.
The 11-25 cassette has a 16 but not the 28 that you probably want but it gives you the "fun" 11 gear, and on a day like 2018 Kona, the right gear for a good 30 minutes of the ride.
The 12-28 cassette has a 16 but not the 11, but it does have the relief 28 that would be more critical on a higher wind day (especially if you want to preserve run legs by spinning more). Does this make sense?

I decided to take my Kona slot for 2019 from IM Louisville (having given up a slot for 2018 and regretting it) and I need some advice.

1) Where to rent, enough for 6 people (friends and other idiot who want to race as well) any favourites that don't require a 2nd mortgage.

Air BnB, VRBO. You've got the towns of Keauhou, Holualoa, and Kona in the general vicinity. If you're out by Keauhou, remember to be patient with traffic when you roll into town for Ironman Village and all of the amazing events that take place during race week. The influx of people in for the race is insane (add to that road closures and the sheer number of people that live on Alii Drive that are seriously just trying to get to work). If you stay out in the Waikoloa area (aka: Wai-ka-blow-a), it gives you a lot of practice with the cross winds and you're closer to Hawi for practice rides. I can't recommend staying at the King Kamehameha hotel or the Kona SeaSide Hotel because the construction guys are up all night putting the finish line together and it's really loud (per my friend that stayed in the King Kam this past year).

2) Take your own bike or rent, travelling with a bike is a b***** or take own wheels/saddle/whatever?

Nothing new on race day: take your own bike.

3) Just how bloody hot is it? I cook at anything above about 70f

Effin hot. And the crosswinds just blow more hot air at you. The temperature feedback on my Garmin when I was "hiking" into town on Alii drive on race day to watch the pros finish this year was up to 106F.

8) Anything else (yeah I know sunscreen)

For the aloha of others and sanity of locals, please, please, please do not bike on the road (left of the white line where the cars are). Use the bike lane. Alii Drive is a single-file ride -- not two abreast. Police have cracked down this year immensely on that as well as people who fly through Stop signs and red lights: people who act like it's 'race day' everyday during the week leading up to the race.
Just ride with Aloha and you'll be good. And practice your shaka for when a driver gives you the right-of-way! 🤙
If you haven't spent too much money at Ironman Village - stay a little longer and explore! This island has Hawaii Volcanoes National Park, night snorkeling with manta rays, and stargazing at 9200ft up at the Mauna Kea Visitor Information Station.

Congrats on qualifying! I would suggest getting set up with accommodations as soon as you can - my understanding is that it fills up fast.

Oh that's cool. Do you know if it works on a Shimano drivetrain? What gear does it skip?