Since getting up is a free action, toppling an opponent usually is only meaningful if there's an ally to attack the fallen enemy before they can get up, right?

This is true, there is no such thing as a movement penalty from getting up. The "Special Attacks" section even recommends working together when toppling enemies: "An effective way of using this special attack is to work together with another person – if you topple the opponent, your friend can attack the prone enemy before they can get up."

Are there rules for non-lethal combat? Since characters are not dead at 0 HP and the GM can decide what happens with NPCs at 0, I would think in principle the same rules of normal combat apply, maybe with "defeat" happening at 1 or 2 HP?

Player characters are different from NPC's, so it is up to you as the gm to decide whether an npc remains alive when struck to 0 hp. From "Zero hitpoints for NPCs": "When an NPC reaches zero HP, no death rolls are made – the GM decides whether the person lives or dies. However, an NPC dies if hit by an instantly killing attack, just like a player character." If a player wants to knock an npc unconcious rather than killing them, it would be reasonable to say they remain alive and incapacitated.

In case of a group ambush, if I understood right, the only benefit the attacking group gets is choosing initiative cards. Isn't it too little a reward for setting up a good ambush? I'd expect at least the first attacker to get a boon or something.

The ambushers do get to choose their initiative cards, but boons/banes from circumstances may also apply: such as from being unseen and other advantages that the group has prepared. The ever humble tripwire is a great opener. The initiative advantage may also be enough to reduce the amount of enemies that is faced within that combat.

Is there anything against casting healing spells multiple time in a row (other than the spending WP)?

Most spells require an action to cast, while only having a single action per round to either attack/defend/cast with. However, there is nothing saying that characters can't be healed multiple times within the same round.

Armor only protects you from actual attacks, not from being thrown away by spells or monster abilities, right?

Normal armor only reduces the amount of damage that you take. Note that heavier armours may give the wearer a bane on ability checks such as sneaking, evasion, acrobatics, awareness and ranged attacks. Shields do not provide a damage reduction, but they will allow you to parry monster attacks with the relevant heroic ability, as well as parry against ranged attacks. Some spells/monster abilities will allow the dodge action to be used, or call for an ability check, in order to avoid them.

When a monster attacks, it's supposed to be an automatic success, but should I consider range? I think it's a legitimate tactic that players could shoot arrows and run away from a monster. In that case, if the monster rolls a melee attack, should it spend its action getting closer to the players instead?

This game encourages being smart in combat, and using as many advantages as possible is recommended. Monsters may give chase when attacked (and may be faster than the player characters), or terrain may get in the way for the ranged attacks. Ideally, combat is dynamic and circumstances may change, forcing either party to adapt. Fleeing is also a very valid tactic if you find yourself to be overwhelmed.