Learn Hardcore German: Anstatt alles sofort zu sagen, lese ich zwischendurch meine Notizen und formuliere dann einen klaren Lösungsvorschlag. - Instead of saying everything immediately, I read my notes in between and then formulate a clear proposed solution.

is treated as position 1 (a single big element). Because that first element already fills the first position, the verb of the main clause must come next:

Anstatt alles sofort zu sagen, lese ich zwischendurch meine Notizen ...

So you get:

First position: Anstatt alles sofort zu sagen
Second position (finite verb): lese
Then the subject: ich
Then the rest: zwischendurch meine Notizen und formuliere dann ...

If you said ..., ich lese ..., the finite verb would be in third position, which breaks the V2 rule for main clauses.

What exactly is the structure Anstatt alles sofort zu sagen? How is anstatt being used?

Here anstatt introduces a zu‑infinitive clause that expresses an alternative / contrast:

Anstatt
- Infinitiv mit zu: anstatt alles sofort zu sagen

Logical full form would be something like:

Anstatt dass ich alles sofort sage, ...

So we have:

Implicit subject (ich) understood from the main clause
Object: alles
Adverb: sofort
zu + Infinitiv: zu sagen

The infinitive clause as a whole functions adverbially: it tells us what the speaker does instead of doing something else.

You can also use statt instead of anstatt here:

Statt alles sofort zu sagen, lese ich ...

That is very common in everyday German and means the same in this context.

Why is there a comma before lese ich? I thought zu‑infinitive constructions sometimes do not take a comma.

German comma rules distinguish between simple and expanded infinitive clauses, and also between certain special words.

Short, simple infinitive clauses may omit the comma:
- Ich versuche(,) das zu verstehen.
But when the infinitive clause is introduced by words like um, ohne, statt, anstatt, außer, als, a comma is mandatory:
- Um das zu verstehen, lese ich den Text noch einmal.
- Anstatt alles sofort zu sagen, lese ich ...

Because this sentence uses anstatt, the infinitive clause must be set off with a comma. So here the comma is obligatory, not optional.

Why is the order alles sofort zu sagen and not sofort alles zu sagen? Are both correct?

Both alles sofort zu sagen and sofort alles zu sagen are grammatically correct.

alles sofort zu sagen focuses slightly more on the entirety of the content (everything) and then adds how: immediately.
sofort alles zu sagen puts a little more emphasis on the immediacy first and then on the scope (everything).

In practice, the difference is minimal; both are very natural. German word order of adverbs and objects is flexible, especially when there is no pronoun involved.

A rough guideline:

Pronouns tend to come earlier.
Adverbs like sofort can move around: alles sofort, sofort alles – both fine here.

What is the function and typical meaning of zwischendurch here, and where can it appear in the sentence?

zwischendurch is an adverb meaning roughly in between / from time to time during something / in the meantime between other activities.

In this sentence it suggests that reading notes happens between other steps in a process, not continuously.

Placement:

Middle field (very common):
- ..., lese ich zwischendurch meine Notizen ...
It can also be moved, depending on emphasis:
- ..., lese ich meine Notizen zwischendurch ... (emphasis shifts a bit)
- Zwischendurch lese ich meine Notizen und ... (fronted for emphasis)

All of these are possible; the version given is very natural and neutral.

Why is it meine Notizen and not meinen Notizen?

The noun Notizen is plural (singular: die Notiz). In the sentence, meine Notizen is the direct object of lese and is therefore in the accusative plural.

For the possessive mein‑ in the plural:

Nominative plural: meine Notizen
Accusative plural: meine Notizen

So meine is correct here.

meinen Notizen would be dative plural (note the extra -n on the noun):

mit meinen Notizen
an meinen Notizen arbeiten

But we do not have a dative preposition here, just a direct object of lesen, so we use meine Notizen (accusative plural).

Why is it einen klaren Lösungsvorschlag and not ein klarer Lösungsvorschlag?

Because einen klaren Lösungsvorschlag is the direct object of formuliere and must therefore be in the accusative singular, masculine.

Vorschlag is masculine: der Vorschlag
In accusative singular: einen Vorschlag
With an adjective after an indefinite article, the adjective takes -en:
- einen klaren Vorschlag

So:

Article: ein → einen (masculine accusative)
Adjective: klar → klaren (weak declension after the article)
Noun: Lösungsvorschlag

Hence: einen klaren Lösungsvorschlag.

ein klarer Lösungsvorschlag would be nominative (subject), e.g.:

Ein klarer Lösungsvorschlag hilft allen weiter.

Why is Lösungsvorschlag written as one word and not Lösung Vorschlag as two words?

German very often forms compounds by joining nouns:

die Lösung (solution)
der Vorschlag (proposal)

Combined into one noun:

der Lösungsvorschlag = literally solution‑proposal, a proposal for a solution.

If you wrote Lösung Vorschlag as two separate words, it would be interpreted as two separate nouns side by side, which is not how German normally expresses “X proposal”. The standard pattern is:

Autobahn (auto + Bahn)
Kaffetasse (Kaffee + Tasse)
Lösungsvorschlag (Lösung + Vorschlag)

So Lösungsvorschlag must be written as a single word.

Is there a difference between dann and danach here? Could I say ... und formuliere danach einen klaren Lösungsvorschlag?

Both dann and danach are temporal; both can work in this sentence, but there is a nuance:

dann = then / after that, very general, often just marking sequence.
danach = afterwards / after that, with a slightly stronger sense of “after the previously mentioned phase is finished”.

In your sentence:

... und formuliere dann einen klaren Lösungsvorschlag.
→ neutral “and then I formulate a clear solution proposal.”
... und formuliere danach einen klaren Lösungsvorschlag.
→ emphasizes “after that phase (of reading notes) I formulate ...”.

Both are correct; dann sounds a bit more neutral and is used very frequently in spoken German.

Why does zu come right before sagen in alles sofort zu sagen? Can anything come between zu and the verb?

In standard German, zu is placed directly before the infinitive:

zu sagen
zu lesen
zu verstehen

Other elements (objects, adverbs) normally go before the zu + infinitive group:

alles sofort zu sagen
das schnell zu verstehen
den Text gründlich zu lesen

You cannot insert normal adverbs or objects between zu and the verb:

✗ alles sofort zu schnell sagen (wrong)
✓ alles sofort schnell zu sagen (clumsy but structurally correct: all adverbs before zu sagen)

For verbs with separable prefixes, zu goes between the prefix and the verb stem:

anzufangen (an + zu + fangen)
nachzudenken (nach + zu + denken)

But zu still stays attached to the infinitive, not to other words around it.

Could I also say Anstatt dass ich alles sofort sage, ... instead of Anstatt alles sofort zu sagen, ...? Is there a difference?

Yes, you can say Anstatt dass ich alles sofort sage, .... It is grammatically correct.

Differences:

Form
- Anstatt alles sofort zu sagen, ...
  → Infinitive clause with zu, subject ich is understood from context.
- Anstatt dass ich alles sofort sage, ...
  → Subordinate clause with dass, subject ich is explicitly stated.
Style
- The zu‑infinitive version sounds more natural and common in modern spoken and written German.
- The dass‑clause can sound a bit more formal, heavier, or old‑fashioned in many contexts, though it is not wrong.

In most everyday situations, learners will be better off preferring:

Anstatt alles sofort zu sagen, ...

and using the dass‑clause only when they specifically want that explicit structure.