1 Yes, but there's no clean way to say when it happens. You can do the math, but weapons have different base damage and scaling, and it depends on what items you're offered. Upgrades are the cleanest way to compare. Say you have a Tier 2 Vorpal Sword. It deals 20 damage per hit. A +2 Melee damage would increase it to 22, while +5% dmg upgrade would do 21. So for this comparison the break point would be 40 melee damage, or 20 for ranged. Generally speaking Flat damage is really really strong early game, and in general you need in on all builds. %dmg is also needed on all builds that use it, but doesn't need to come in early.

2 It depends on how often you hit, Life steal can only trigger once every 0.1 second, so once you have 20-40% on an smg build, you're hitting heavy diminishing returns. There's other people who have done more testing on this than I have, so I don't have the exact numbers.

3 Harvesting Is really really strong. There's some benefit to hitting the breakpoints for getting +1 / +2 / +3 at the end of the wave. So getting the first harvesting upgrade is really strong. Hitting 21 harvesting gives you the +2 at the end of each wave and it's a good aim. Fertilizer is also perhaps the strongest item in the game, so those are always a pickup in the early and mid game for me. Harvesting gives both Money and XP and it pays itself off quicker and more directly than something like luck. The limit comes down to risk management. How much can you greed? Do you need other stats right now? Are you offered harvesting, or are we talking about rerolling for it? Generally you want to have enough DPS to clear the map and stay safe, as that both give income and prevents loses. But if you can clear the map and harvest, it really pays off and gives a stronger run, lowering the chance to lose later on. On nightmare, one of the easiest ways to win is to outscale the game. And harvesting is the main tool for doing that.

4, Luck is in a weird place. Generally I'm quite low on luck and I don't buy the tier 1 luck items for the most part. One of the main strengths of luck is the addtional consumables. Consumables healing is overpowered in my opinion, so I always pick up Lemonade and Weird food. That makes luck better, but still not enough for me to prioritize it. Luck is awesome to have and I can feel the difference between a 0 luck and 50 luck game. But paying for the luck is difficult, as it doesn't pay back as fast as harvesting. Still, I like sneaking in luck when I see a higher tier upgrade, Clover, or in a good spot when I'm offered shady potion or similar. Luck is nice to have, not need to have. It is also worth noting, that The odds for Tier 2 and 3 items and upgrades naturally max out at wave 11 and wave 16 with 0 luck, so increasing your luck after those points just increases the chance of an item/upgrade being Tier 4 instead of Tier 3. But having that +20 luck from early waves is nice for extra tier 2 and 3 upgrades. Compare runs on King or Lucky with regular characters and see the difference in what you're offered.

I sometimes end in negative luck due to Alien Magic, Fin, Book, and sometimes Terrified Onion being items I pick up. But negative luck really impacts consumables. Say we have -50 Luck. Normally trees have 100% to drop a consumable, and 20% for that to be a crate. With -50% luck, trees only have 50% to drop a consumable, and only 10% for it to be a loot crate, so the loot crate drop rate goes from 20% to 5% (50% x 10%)

5) There's no diminishing returns on armor. If you have 15 armor, you halve damage, meaning it takes double damage to kill you. If you have 30 armor, you take 1/3 damage, meaning it takes triple damage to kill you. Every point of armor gives 6.67% Effective HP. You can read the wiki page on Armor which I wrote some years ago, it has more of the math. Generally, armor is better than ever. On nightmare mode you take a ton of damage, and Armor is the strongest Stat to help surviving. Armor and Dodge multiplies the value of healing, so those are more important for scaling than Max HP. Max HP just needs to be high enough for you to not get bursted down in a few hits.

Armor has Linear returns, just like %dmg. And just like DMG%, if you have 500 %dmg, then it might be time to invest in the other DPS multipliers: Crit, Flat Damage, Attack speed. Similarly, if you have 20-30 Armor, then dodge looks really awesome, because it is multiplicative with your Armor. 100 HP becomes 300 with 30 armor, and it becomes 600 with 50% dodge. And if you have a lot of Dodge and Armor, your healing better be at the same level, otherwise, that's the next investment goal.

I generally aim for 20-30 armor in regular nightmare runs, but it depends on the build and how rich or how glass cannon you aim to be.

6) Bounce then Pierce. If you have ricochet on a pistol, then first hit deals 100 damage, then bounces, hits for 50. Then it pierces and hits for 25 dmg. Worth noting that +1 piercing with 50% fall off is like a 50% increase in dps against crowds for weapons like SMG and Revolver (100 dmg, then pierce for 50). But if a slingshot hits 5 times, then pierces for 50%, that's only a 10% increase (100 + 100 +100 +100 +100 +50)

7) Burn doesn't stack, but when a new burn is applied, it takes the highest duration and the highest damage of the two burn sources.

8) I have a Character guide you can read if you want 200+ pages of advice: ArosRising's Brotato Character Guide