Hi Senpai,
I'll discuss with you if you like :)
How much does one attack slot do?
It depends on the hero. Some hero have high base dmg, while others, low. Any attack slot for heroes that already deal high dmg, will see a huge spike in power. While other heroes that have very low dmg (like alex for example) won't see any benefit from atk slot.
Also we know mathematically how much potential each slot can do. It's a good enough indicator of how effective any attack slot can provide to each hero.
Segway into your next question, we individually have to determine if any other slot will outweigh the benefits of an atk slot.
When it gets replaced by a def or func slot, which would be better?
It all depends for each hero. Though if you want my general rule of thumb:
Though personally, i still strive for double atk/atk for these class. I don't settle for 2nd best.
What's the best non-attack upgrade?
Are pallys and priests held back by their lack of good upgrade options on their 6* weapon?
I think Paladins are held back the most. Cause healers can still make good use of atk slot. However, Paladins have such dismal base attack, and usually their passive functions with their def slot. (exceptions are vane/kriem ofc)
Most healers are fine with atk slots, though there are obviously some that heal based off of HP/Ally atk/enemy atk. For these healers, atk is completely useless, so these particular healers will suffer the same as paladins.
Most get by just fine imo, but that's because they have good heroes there to begin with.
What about warriors and archers? Would they benefit from having a devil hunter or superior staff?
I don't particularly mind Answerer or Red Falchion. But to answer your question here's my idea.
Susanoo, Monte. Sigruna, Gon, R-0, Gip, Arona. Though to be perfectly honest here, the func slot is fine too. CC if you want dmg, lifesteal if you wanna sustain more.
What if everyone had to use fantasy harmonys/volcanic hammers? What about answerer/red falchion?
Would any heroes be better or worse?
How important are the upgrade options to each class?
Having the right upgrade is important to every hero to perform their job efficiently. The most important ones are pala/healers. Though gunner/archers/mage also need good upgrade too. The only ones that can kinda work even with bad slots are warrior. Though it may not be optimal.