Does it boil down to this?

I don't think cp matters that much. I rarely see gyms attacked and not taken down completely. so it usually doesn't matter if you're at the top or bottom.

IV can make as much as 300cp difference to overall CP though so it is important, especially as most people have the higher tier pokemon now. It comes down to how good you're dragonite or snorlaxs IVs are, and your level of course .

From what I understand, training your attackers is beneficial if you will be attacking enemy gyms with them. Therefore, IVs are important for them. It's nice to have also have some lower level Pokemon for training gyms of your team. For these, IVs are unimportant. The article listed here shows how prestige is calculated when attacking and training gyms.

https://pkmngotrading.com/wiki/Gyms

IVs are, for the most part, trophies. As an attacker it may in a few battles account for an extra potion or so here and there. Move sets account for the majority of the difference between a good and bad pokemon. The difference in dps and how they will perform in battle will vary widely based on moveset, IVs you need to break out a calculator to see the difference, its really that small. On spreadsheets and sims IVs seem to be moderately important assuming you arent comparing 0 ivs to 15 ivs. In the real world once you add type advantage and match ups, lag, dodge and game mechanics it really is laughable how little of a difference it makes.

It depends on what your intent is.

If your intent is just to stay in a gym, then sure, obviously who has the pokemon with the highest cp is going to stay there longest, even if it's only a few minutes. But, remember, a gym full of solid pokemon stands a better chance of staying together as a whole. Therefore, if a gym can stay standing as a whole, then your specific position in it, does not matter as much.

Honestly, for attackers, the most crucial thing is potions and revives. Even 6 Raticates can take down a Dragonite. If you have the resources to keep your pokemon alive, I do not think ivs or moveset, or even what you are attacking with matters that much. Now if you are trying to do it strictly efficiently, then sure, you can get into movesets and type advantage, but ultimately if you have time and resources, then you can take any gym.

First, I agree with you on IVs not being very important for attackers. I would still power up main attackers to a reasonable level, but no need to touch your secondary attackers.

The gym defense is where I disagree, but this comes down to what you are going for. If your sole purpose is to stick a worthless pokemon in a gym for as long as possible, you are correct. But those gyms are very weak gyms. I prefer to add pokemon to a gym that make it stronger. It doesn't matter as much if my pokemon is towards the bottom because if it is annoying enough, it can deter people right there, and a stronger gym will last longer. I always take gyms all the way out, the question is how long will it take me. Others are not the same as I am. I can see 3 gyms from my house and so many times the gyms have been at level 3-4 with my acid/blizz Tentacruel or confusion/ice beam Slowbro towards the bottom. These gyms get attacked and knocked down a level and then people leave them alone.

I would focus on building a strong, well-rounded gym. My pokemon that has been in a gym the longest is actually my 1860 cp Nidoqueen. The cps of the pokemon in that gym aren't great, but it never goes down more than a few spots. My Dragonite has been knocked out of the top spot of a level 10 gym 3 times in that same time. Build a good gym in a good location, don't just throw Dragonites in a gym...but every area is different.

One thing that I rarely seen mentioned is time: I can't be the only person who doesn't have the patience (or time, especially during the workday) to stand around for 15 minutes to take out a single gym. Having higher CP attackers can help speed up gym battles, at least to a degree. For example, until recently all of my top attacking pokemon were vaporeons; they're quite strong and durable, but OMG it takes forever to kill anything with a vaporeon (except fire types). I powered up a couple of my vaporeons to max CP for my level and I noticed that shaved a few seconds off of each battle. I consider that worth the stardust investment.

First, u need to reach max level.
END.

1 - You're right.
2 - Well, I don't agree. A level 30 Dragonite, with 90% IV, can reach +/- 300 cp more than a 0% IV one. So, in the end, IV matters. About the moveset, it matters more than IV, how many times did you see the gym defender with "Battles Won: X", probably he could lost the battle if the moveset was garbage.
3 - Dragonite is one of the best defenders, no doubt, but a Snorlax with less CP can defeat an attacker that a Dragonite doesn't, and make the player stop attacking, cause he's gonna waste so many time and resources, that it's not gonna worth.
4 - It's a good strat, but it works better if your Dragonites have a good moveset, as I said before.
5 - Agreed, since you can dodge attacks, DEF and HP IV's doesn't really matters that much, but a good ATK IV is nice to have, since it can accelerate the process.
6 - Moveset > all, always.