Learn Hardcore Icelandic: Framhaldið verður líklega erfiðara, en við munum hafa lært mikið. - The continuation will probably be more difficult, but we will have learned a lot.

If you used erfitt, that would just be “difficult” (not “more difficult”).
If you used erfiðari, that would be the wrong gender, because framhald(ið) is neuter, not masculine/feminine.

What is the role of líklega, and could it go somewhere else in the sentence?

Líklega is an adverb meaning “probably / likely”.

In Icelandic main clauses, the typical word order is:

Subject
Finite verb
Sentence adverb (like líklega)
Rest of the predicate

So:

Framhaldið (subject)
verður (finite verb)
líklega (adverb)
erfiðara (rest)

This is the most natural position.

Other possible positions:

Líklega verður framhaldið erfiðara.
(“Probably the continuation will be more difficult.” – adverb at the very beginning.)

Putting it in odd places like Framhaldið líklega verður erfiðara is not natural in standard Icelandic.

What tense is verður, and how does Icelandic express the future here?

Verður is the 3rd person singular present tense of verða (“to become / to be, in future sense”).

Icelandic usually uses present tense with a future meaning, so:

Framhaldið verður erfiðara
literally: “The continuation becomes more difficult”
but in context: “The continuation will be more difficult.”

You don’t need a separate future tense marker (like “will”) with verða; the present form verður naturally has a future meaning here.

What exactly is munum hafa lært – what tense is that and how is it formed?

Munum hafa lært is a future perfect construction:

munum – 1st person plural present of munu (a modal/auxiliary meaning roughly “will”)
hafa – infinitive of “to have”
lært – past participle / supine of læra (“to learn”)

Together:

við munum hafa lært mikið
= “we will have learned a lot”

So the pattern is:

munu (present) + hafa + past participle
= will have done something (future perfect)

What is the difference between við munum læra mikið and við munum hafa lært mikið?

Both refer to the future, but they differ in time perspective:

Við munum læra mikið.
= “We will learn a lot.”
Focus: the learning itself in the future.
Við munum hafa lært mikið.
= “We will have learned a lot.”
Focus: by some later future point, the learning will already be completed.

Example:

“Next year, the course will be harder, but by then we will have learned a lot.”
→ fits við munum hafa lært mikið, because you’re looking back from a future point.

If you just mean “In the future we will learn a lot”, við munum læra mikið is enough.

Why is mikið used after lært, and is it an object or an adverb?

Mikið comes from the adjective mikill (“much, big, great”).

In this sentence:

lært mikið = “learned a lot / learned much”

Here mikið behaves like a quantifier / object-like word: it tells you how much has been learned. It’s in the neuter singular form, which is the standard form when mikill is used in this kind of “amount” meaning.

So structurally, you can think of it as:

læra eitthvað = “to learn something”
læra mikið = “to learn a lot (of things)”

It’s not an adverb like “well”; it’s more like a vague direct object meaning “much / a lot (of stuff)”.

Why is there a comma before en in …, en við munum hafa lært mikið?

In Icelandic, you normally put a comma before “en” when it joins two main clauses:

Framhaldið verður líklega erfiðara – first main clause
við munum hafa lært mikið – second main clause
Joined by en (“but”), so a comma is used:
Framhaldið verður líklega erfiðara, en við munum hafa lært mikið.

This is similar to older or more formal English punctuation:

“The continuation will probably be more difficult, but we will have learned a lot.”

Both en and heldur can mean “but”. Why is en used here and not heldur?

Both can translate as “but”, but they are used differently:

en = the normal “but”, used in most contrast sentences.
heldur (as a conjunction) is usually “but rather / but instead”, often after a negation.

Examples:

Ég vil ekki te, heldur kaffi.
“I don’t want tea, but (rather) coffee.”

In our sentence there is no negation and we just contrast two facts:

The continuation will be harder
but
we will have learned a lot.

So the normal en is appropriate:

Framhaldið verður líklega erfiðara, en við munum hafa lært mikið.

Is the word order in both clauses fixed, or could we move things around?

The sentence mostly follows standard Icelandic main-clause word order:

Framhaldið – subject
verður – finite verb (in 2nd position)
líklega – sentence adverb
erfiðara – complement

Second clause:

við – subject
munum – finite verb (2nd position)
hafa lært mikið – rest of the verb phrase

You can change word order a little, but some changes sound unnatural or wrong:

✔ Líklega verður framhaldið erfiðara, en við munum hafa lært mikið.
(Adverb moved to the front for emphasis.)
✖ Framhaldið líklega verður erfiðara…
This breaks the rule that the finite verb normally stays in second position in main clauses.

So, generally:

Keep the finite verb in 2nd position.
Put adverbs like líklega right after the finite verb, unless you deliberately move them to the very beginning for emphasis.