Publikasinya sedemikian jelasnya sehingga banyak donatur datang.

They do two different jobs.

• publikasinya: here -nya most naturally makes publikasi feel definite, like the publicity / the publication / its publicity, depending on context.
• jelasnya: here -nya is not possessive. It adds emphasis in a pattern like sedemikian + adjective + -nya, which means so ... or to such a degree ...

So:

• publikasinya = the/its publicity
• sedemikian jelasnya = so clear, so very clear, clear to such an extent

The second -nya is part of an emphatic style and is especially common in more formal or written Indonesian.

Sedemikian means so, such, or to such an extent.

In this sentence, it works with sehingga to create a cause-result pattern:

• sedemikian ... sehingga ...
• so ... that ...

So the sentence structure is basically:

• Publikasinya sedemikian jelasnya sehingga banyak donatur datang.
• The publicity was so clear that many donors came.

This is a fairly formal way to express that idea.

Because they form a pair.

A very common formal pattern in Indonesian is:

• sedemikian + adjective/adverb + sehingga ...
• so + adjective/adverb + that ...

Examples:

• Ia berbicara sedemikian pelannya sehingga saya tidak mendengar.
  • He spoke so softly that I could not hear.
• Masalahnya sedemikian rumit sehingga sulit diselesaikan.
  • The problem was so complicated that it was hard to solve.

So in your sentence:

• sedemikian jelasnya = so clear
• sehingga banyak donatur datang = that many donors came

Jelas by itself would already be grammatical:

• Publikasinya sedemikian jelas sehingga banyak donatur datang.

That version is fine and probably a little simpler.

Using jelasnya adds emphasis and makes the style more literary or formal. It strengthens the sense of degree. So:

• sedemikian jelas = so clear
• sedemikian jelasnya = so very clear / clear to such an extent

It is not a completely different meaning, but it is a stronger, more stylized way of saying it.

In Indonesian, publikasi can mean publication, but in many contexts it can also refer more broadly to publicity, promotion, or the act/process of making something public.

So the exact English translation depends on context.

In this sentence, because it is followed by banyak donatur datang, publikasinya may sound like:

• the publicity
• the promotion
• the way it was publicized

rather than just a physical publication like a book or journal article.

A learner should not assume Indonesian publikasi always matches English publication one-for-one.

Yes, it sounds fairly formal, especially because of:

• sedemikian
• jelasnya
• the overall sedemikian ... sehingga ... construction

In everyday speech, many speakers would choose something simpler, such as:

• Publikasinya sangat jelas, jadi banyak donatur datang.
• Promosinya begitu jelas sampai banyak donatur datang.

Those are more conversational.

Your original sentence is natural, but it has a written, polished, or rhetorical tone.

Yes, but the structure changes a little.

Possible alternatives:

• Publikasinya begitu jelas sehingga banyak donatur datang.
  • very natural; so clear that
• Publikasinya sangat jelas sehingga banyak donatur datang.
  • grammatical, but sangat is more like very, so the result clause feels slightly less tightly linked than with begitu or sedemikian
• Publikasinya jelas sekali, sehingga banyak donatur datang.
  • also possible

The closest replacement for sedemikian in this pattern is begitu:

• begitu ... sehingga ...
• sedemikian ... sehingga ...

But sedemikian is more formal.

The subject is banyak donatur.

So:

• banyak donatur datang
• many donors came

Here banyak means many, and donatur is the noun it modifies.

This is a normal Indonesian noun phrase:

• banyak orang = many people
• banyak siswa = many students
• banyak donatur = many donors

Then datang is the verb: to come.

Because datang means to come, focusing on the donors’ arrival.

• banyak donatur datang = many donors came

By contrast, mendatangi usually means to come to / visit / approach a place or person and normally takes an object.

For example:

• Banyak donatur mendatangi kantor yayasan.
  • Many donors came to the foundation’s office.

So if the sentence just wants to say that donors came as a result of the clear publicity, datang is the natural choice.

Some changes are possible, but not all of them sound equally natural.

The original order is good:

• Publikasinya sedemikian jelasnya sehingga banyak donatur datang.

A simpler variation:

• Publikasinya sedemikian jelas sehingga banyak donatur datang.

You could also front the result clause for style, but that would be less straightforward and less common for learners.

What you usually should keep is the basic relationship:

• cause/degree first: publikasinya sedemikian jelasnya
• result second: sehingga banyak donatur datang

That order makes the sentence easy to process.

It can cover both ideas, depending on context.

Jelas is a broad adjective that can mean:

• clear
• plain
• obvious
• easy to understand

In this sentence, publikasinya sedemikian jelasnya most likely means the publicity or message was presented clearly enough that people understood it and responded.

So in natural English, you might choose:

• The publicity was so clear that many donors came.
• The message was communicated so clearly that many donors came.

The exact nuance depends on what publikasi refers to in the larger context.