[RPG] Is it easier to wake a character from ‘mundane’ sleep than from ‘magically induced’ sleep

This spell sends creatures into a magical slumber. Roll 5d8; the total is how many hit points of creatures this spell can affect. Creatures within 20 feet of a point you choose within range are affected in ascending order of their current hit points (ignoring unconscious creatures). Starting with the creature that has the lowest current hit points, each creature affected by this spell falls unconscious until the spell ends, the sleeper takes damage, or someone uses an action to shake or slap the sleeper awake.

A sleeping creature is considered unconscious (as the spell's effect suggests). Therefore, your sleeping target is not affected by the spell.

Does damage resolve (waking a sleeping target) between each individual attack?

The damage resolves on a per attack basis

In the PHB between pages 193 and 194 is "Making an Attack" header, which specifies the order of an attack. Resolving the attack is when you deal damage.

Chapter 9: Combat, Making An Attack: You make the attack roll. On a hit, you roll damage...

If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack

For instance, with Extra Attack at level 5, you're making 2 attack rolls and therefore 2 instances of damage. If they're unconscious from sleep, the first attack automatically crits if you hit their AC, then they wake up, and the second attack does not automatically crit. It's important to note that you only get automatic crits on an unconscious target if you are within 5 feet of it, and the same applies for advantage on prone targets.

The same logic applies for spells, any time you make a spell attack roll, you're doing an instance of damage. Spells that add to your attack damage, like smite spells, add on to the initial damage, so that damage is a crit as well for the first attack.

For things like Magic Missile, I've written an answer on another question that may be helpful.

For spells without attack rolls, you need to look at it on a case by case basis. I can't think of a spell besides magic missile that would appear to have multiple instances of damage within itself that are not separated by time or triggers.

Is there a difference between abilities granting multiple attacks as part of the same action vs. a different action (such as bonus action)?

The same logic applies as above: if you're making separate attack rolls, you're making separate attacks. In addition, bonus actions and actions happen at different points in time. See page 189 of the PHB

Chapter 9: Combat, Bonus Actions: You choose when you take a bonus action during your turn...

If you were granted an attack or spell with a bonus action that dealt damage, it would crit (if within 5 feet of the sleeping target), and any actions after that would not automatically crit. Once again, smite spells like Thunderous Smite are cast as a bonus action, but only trigger damage once you hit with a melee weapon and their damage is added, which makes it a part of your melee attack damage. The first attack with smite will crit, so double all of your damage dice, including the smite dice.

Side note Because of questions like these, I highly recommend rolling your attacks and damage separately. Some players are tempted to roll all attacks together and then roll their damage all at once for ease of use (or sometimes roll damage once and double/triple the damage! The horror!), and cutting corners like that can, in this case, miscalculate critical damage or deal too much damage to one person (you can split your attacks up between movement!).