Q about IVs

It depends on the damage breakpoints. PoGo uses a flooring formula for damage per fast move.

For example, Dragonite with 15 attack stat and Dragon Rage fast move (just for example, this move doesn't exist in PoGo) deals 7.98 damage per hit against a perfect level 39 Blissey at level 29. This is rounded down to 7 damage.

At level 29.5 with 15 attack stat, it deals 8.03 damage, which is now rounded to 8.
But at level 29.5 with 14 attack stat, it deals 7.99 damage, which is still rounded down to 7.

So, the 1 less attack stat means 12.5% less DPS from the fast move, which can be a big difference in soloing T3 raids.

To reach 8 damage, the Dragonite with 14 attack stat will need to be powered up once more, using an additional 5000 stardust.

If the attack stat is lower, let's say 10, the Dragonite will need to be powered to, let's say 32 to deal 8 damage per hit. This means 27000 extra stardust for powering up compared to the one with 15 attack.

No, no significant difference but it's a good excuse to power up a 100 or 98 IV mon.

It depends on your personal goals.

Under the old arena system, iv really mattered for the top defenders, not because they got noticeably better with higher ivs but because they could reach a higher cp value. This point is moot after the arena re-work.

Under the new system, better ivs increase survivabilty and attack power, but this is relative in two ways: first, often times a mon with lower ivs can reach the same (or so close it does not matter) attack power and survivabilty as a higher iv one, although it might need a few additional power ups to do so. Second, unless you go for level 3 solo raids where every second matters, you will hardly notice a difference between an average and a high iv mon in most group raids. There is one of course, but it will hardly ever make the difference between winning or losing or getting a higher or lower damage bonus.

There are a few notable exceptions to this however: if you have an attacker with a low damage quick move (e.q. snorlax with lick) which due to low attack iv stays below a quick attack break point that the very same attacker could reach at his level if he had a higher iv, this will noticeably lower this mon's performance in raids (which wouldn't be the case for e.g. machamp, where the difference between quick attack break points is way smaller). Similarly, low sta/def ivs can in extreme situations seriously lower a mons usefulness, but this is really a rare exception (classic example: gengar vs mew2 with confusion).

And then there is the last reason for aiming at high ivs: collector's pride. While I understand this motive in principle, I often wonder whether many of these players who boast on this forum that they would never power up a mon below 9x % total iv might only be able to afford this attitude because they use trackers or don't play raids competitvely.

As to your last question, you will find the answer above: the more damage a quick move produces, the less relative difference reaching the next higher break point makes. This is also the reason why on this forum it is usually the quick move break points that are being discussed - the (high damage) charge move break points just matter a lot less for a pokemon's overall combat performance.

I suggest to reframe the question from strength, which is hard to judge, to levels. A level 30 Dragonite with perfect IVs is equivalent to a crappy one at level 38. Which will cost you 140 candies extra and 116.000 stardust. The difference between a perfect Dragonite and a 90% one is still about one level. At level 30 this is only 10 candy and about 10.000 stardust. At level 38 it is already 27 candy and 19.000 stardust.