Baited Hook

Yes. "Kill a friendly unit" comes after the ":" in Baited Hook's activated ability, so it is an effect instruction rather than part of the activation cost.^377.1 That instruction targets a friendly unit, and you choose that target while the ability is being finalized.^355.10403.2

After Baited Hook's ability is finalized, opponents have an opportunity to play Reactions before it resolves and kills the target.^407.4 The later might limit refers to the unit affected by the kill, so these are linked instructions.^359.3.e.14 After the unit enters the trash, Baited Hook uses its might from immediately before that zone change.^359.3.e.13

The unit is not killed, you still look at the top 5 cards, but you cannot banish or play a unit from among them and must recycle every card you looked at.

If the friendly unit is no longer a legal target when the ability resolves, it is unaffected and the kill instruction is ignored.^{359.3.e.2359.3.e.5} Its might is then null, so the instructions that would use that might to banish and play a unit are ignored.^359.3.e.12 The independent instruction to recycle the rest still happens, and no card was banished, so every card you looked at is recycled.^359.3.e.5

No. You still look at the top 5 cards of your main deck, but you cannot banish or play a unit from among them and must recycle every card you looked at.

When Mordekaiser, Twice Slain replaces his death, Baited Hook's kill action does not occur.^370.1.a.1 The banish instruction directly references that action through "the killed unit," so it is ignored when the kill is negated.^{359.3.e.6359.3.e.14359.3.e.14.b} The separate instruction to "play it" is also ignored because the banish instruction it refers to was ignored.^{135.2.b.5359.3.e.14.a}

The independent instruction to look at the top 5 cards still executes because it does not refer to the negated kill.^359.3.e.14.b The recycle instruction refers to the cards you looked at rather than the kill, so it also executes.^{359.3.e.14359.3.e.14.a} Because none was banished, every card you looked at is "the rest" and is recycled.

The unit played by Baited Hook enters before the killed unit's [Deathknell] resolves. Abilities triggered by playing that unit also resolve before the [Deathknell].

Finalizing first does not mean resolving first: pending items finalize in the order they were added to the chain,^337.1.b but the newest finalized item resolves first.^340.1

1. Baited Hook kills the unit and adds its [Deathknell] as a pending item, then starts playing the chosen unit and adds it after the [Deathknell].^{808.1.d.2354.2}

2. The [Deathknell] finalizes first and waits on the chain. The chosen unit finalizes next and enters immediately, adding any abilities triggered by playing it above the [Deathknell].^{337.2359.2.c383.2.c383.3383.4.a.2}

3. The newer triggered abilities resolve first, followed by the older [Deathknell].

No. Baited Hook kills Glasc Mixologist before it plays Karthus, Eternal. Glasc's [Deathknell] triggers before he enters the trash.^808.1.d.2 Karthus's passive ability is active only while he is on the board.^365.1 Karthus entering later does not reevaluate Glasc's earlier death or make the [Deathknell] trigger again.^383.2.c.3 Glasc's [Deathknell] can play only one unit from the trash.