Se mi estus sciinta la kaŭzon pli frue, mi estus ŝparinta tempon kaj jam elektinta recepton por la kuko.

Because estus sciinta is the normal way to express an unreal past meaning like would have known.

• estus = the conditional form of esti (to be)
• sciinta = the active past participle of scii (to know)

So:

• mi scius = I would know
• mi estus sciinta = I would have known

In this sentence, the speaker is talking about a past situation that did not really happen, so the compound form is needed.

Esperanto does not copy English tense patterns exactly. In a past contrary-to-fact sentence, it is normal to use the conditional construction in both parts:

• Se mi estus sciinta ...
• mi estus ŝparinta ...

This shows that both the condition and the result are unreal and belong to the past.

So even though English uses had known in the if-clause, Esperanto commonly uses estus sciinta there.

They are all active past participles with -int-.

• sciinta = having known
• ŝparinta = having saved
• elektinta = having chosen

When combined with estus, they give a meaning like would have ...-ed:

• estus sciinta = would have known
• estus ŝparinta = would have saved
• estus elektinta = would have chosen

The ending -int- shows that the action is viewed as already completed relative to the reference point.

Because the same estus applies to both participles:

• mi estus ŝparinta tempon kaj jam elektinta recepton

This is understood as:

• mi estus ŝparinta tempon kaj estus jam elektinta recepton

Esperanto often omits a repeated auxiliary when it is clearly shared. English does the same kind of thing:

• I would have saved time and already chosen a recipe

They are direct objects, so they take the accusative ending -n.

• scii la kaŭzon = to know the cause
• elekti recepton = to choose a recipe

So:

• la kaŭzon = what was known
• recepton = what was chosen

This is a very common feature of Esperanto grammar: the direct object gets -n.

Because it is not a direct object. It comes after the preposition por:

• por la kuko = for the cake

After a preposition, Esperanto normally does not use the accusative unless there is some special reason. Here, la kuko is simply the object of por, so it stays without -n.

Jam means already.

Here it adds the idea that, in that hypothetical past situation, the speaker would not only have saved time but would also already have chosen a recipe by then.

So jam emphasizes that the choosing would have been completed at that earlier point.

Pli frue means earlier, so it compares one time with another.

The idea is:

• earlier than I actually learned it
• earlier than I did in reality

If you said just frue, that would mean early in a more general sense. But pli frue specifically shows comparison.

Because por shows purpose or intended use:

• recepto por la kuko = a recipe for the cake

That means a recipe intended to make that cake. This is the natural preposition here.

Yes, to some extent. Esperanto word order is fairly flexible because endings show the grammatical roles clearly.

For example, these are possible:

• Se mi pli frue estus sciinta la kaŭzon...
• mi estus jam elektinta recepton...

But the original version is natural and clear:

• Se mi estus sciinta la kaŭzon pli frue, mi estus ŝparinta tempon kaj jam elektinta recepton por la kuko.

So the word order can move a little for emphasis, but the given sentence is a good standard model.

Not if you want the same meaning.

• Se mi scius ... usually means If I knew ... / If I were to know ...
• Se mi estus sciinta ... means If I had known ...

Your sentence is about a past unreal situation, so estus sciinta is the better choice. It makes it clear that the knowing would have happened before the other hypothetical past results.