Новая грамматическая тема: частица «пусть» может выражать уступку: «Пусть тыква и сладкая, суп из неё всё равно может быть очень вкусным».

Not in this sentence. Here пусть is a concessive particle. It means something like granted, even if, or although.

So Пусть тыква и сладкая... means the speaker is admitting one point first, and then giving a main point that still stands.

This is different from the other common use of пусть, where it means let:

• Пусть он войдёт = Let him come in

In your sentence, пусть is not giving permission or a command. It is introducing a concession.

Here и does not really mean and.

In this pattern, и helps emphasize the concessive idea:

• Пусть тыква и сладкая, ...

It has the sense of:

• Granted that pumpkin is sweet, ...
• Even if pumpkin is sweet, ...

So и marks the quality being admitted before the contrast. It makes the structure sound more natural and idiomatic.

Without и, the sentence may still be understandable, but this pattern very often includes it.

Because it agrees with тыква.

• тыква is a feminine singular noun
• so the adjective must also be feminine singular:
  • сладкая

This is just normal adjective agreement in Russian.

Всё равно means something like all the same, still, or nevertheless.

It reinforces the contrast between the concessive clause and the main clause:

• Пусть тыква и сладкая, суп из неё всё равно может быть очень вкусным.

The idea is:

• Even if pumpkin is sweet, the soup can still be very tasty.

It is not always absolutely required, but it is very natural in sentences like this because it makes the opposition clearer and stronger.

Because after быть and similar verbs, a full adjective used as a predicate is often put in the instrumental case.

Here the phrase is:

• может быть очень вкусным

So вкусным is instrumental singular masculine, agreeing with суп.

This is very common after verbs like:

• быть = to be
• стать = to become
• оказаться = to turn out to be

Compare:

• Суп вкусный = The soup is tasty
  
  • no explicit быть in the present tense
• Суп может быть вкусным = The soup can be tasty
  
  • after быть, instrumental is standard

The preposition из requires the genitive case.

So:

• она becomes неё after из
• из неё = out of it / from it

There are two things to notice:

1. из takes the genitive
2. After many prepositions, pronouns like она gain an initial н:
   • у неё
   • к ней
   • из неё

So из её would be wrong here.

Often, yes.

For example:

• Хотя тыква и сладкая, суп из неё всё равно может быть очень вкусным.

This is also natural.

The difference is mainly in nuance:

• хотя = a straightforward although
• пусть = granted that, admittedly, even if

So пусть can sound a little more like the speaker is accepting an objection and then moving on to the real point.

They are often close in meaning, but пусть has a more specific concessive flavor.

No. It introduces a clause, but in Russian the verb быть is often omitted in the present tense.

That is what happens here:

• Пусть тыква и сладкая...

This is understood as something like:

• Пусть тыква и есть сладкая...

But Russian normally does not say есть in this kind of present-tense statement, so the sentence is perfectly normal as it is.

So the structure is complete even though there is no visible present-tense form of to be.

Concessive пусть can be followed by a normal finite clause in different tenses.

For example:

• Пусть он и опоздал, мы всё равно начали без него.
• Пусть она и молода, опыта у неё достаточно.
• Пусть завтра и будет дождь, мы всё равно поедем.

So this use of пусть does not require an infinitive, and it is not an imperative form. It simply introduces a conceded fact.

Because Пусть тыква и сладкая is a concessive clause, and it is separated from the main clause by a comma:

• Пусть тыква и сладкая, суп из неё всё равно может быть очень вкусным.

The comma marks the boundary between:

1. the conceded point
2. the main statement

This is the normal punctuation for this kind of complex sentence in Russian.