Пусть духи и лёгкие, в закрытой комнате запах становится сильнее, поэтому в салоне их лучше не использовать слишком много.

Here пусть is not the command-like let meaning. It is being used in a concessive sense: even if, though, or granted that.

So Пусть духи и лёгкие, ... means something like:

Even if the perfume is light, ...
or
Though the perfume is light, ...

This is a fairly common Russian pattern:

Пусть X, Y = Even if X, Y
Пусть он и устал, он работает = Even though he is tired, he works

Because духи is one of those Russian nouns that is grammatically plural only in this meaning. It means perfume or perfumes, but Russian treats it as plural.

That is why other words agreeing with it are also plural:

• духи
• лёгкие
• их

Important: the singular дух does not mean one perfume. It usually means spirit, breath, or something similar depending on context.

So you should learn духи as a fixed vocabulary item meaning perfume.

No. Here лёгкие is an adjective, the plural form of лёгкий, meaning light, mild, or not heavy/strong.

It agrees with духи, which is grammatically plural:

• лёгкие духи = light perfume

You are right that лёгкие can also be a noun meaning lungs, but that is a different use of the same form. Context makes it clear here that it is an adjective.

Because adjectives in Russian must agree with the noun they describe in number, gender, and case.

Since духи is plural, the adjective must also be plural:

• singular masculine: лёгкий
• singular feminine: лёгкая
• singular neuter: лёгкое
• plural: лёгкие

So:

• лёгкие духи = correct
• лёгкий духи = incorrect

Not really. In this pattern, и is part of a concessive construction and does not simply mean and.

Пусть духи и лёгкие, ... is close to:

Хотя духи и лёгкие, ... = Although the perfume is light, ...

So the и helps emphasize the contrast:

Even though they are light, still...

This use is very common after words like хотя, пусть, and similar concessive structures.

No, because запах refers to the smell/aroma as one overall thing.

So the logic is:

• духи = perfume, grammatically plural
• запах = the smell/scent, singular

Russian is saying that the perfume may be light, but the smell in a closed room becomes stronger. That is naturally expressed with singular запах.

Становится is present tense imperfective, and Russian often uses the present tense to describe a general truth, habitual result, or what typically happens.

So here it means something like:

in a closed room, the smell gets stronger

That sounds like a general observation.

If you used станет, that would sound more like a specific future event:

the smell will become stronger

So становится is the better choice for a general statement.

Because both phrases describe location, not movement.

With в:

• в + accusative usually means motion into
• в + prepositional usually means being in a place

Here we have location:

• в закрытой комнате = in a closed room
• в салоне = in the salon / cabin / interior

Forms:

• комната → в комнате
• закрытая → в закрытой
• салон → в салоне

So the case choice is normal and expected.

Because использовать takes a direct object, and direct objects are not in the nominative case.

• они = nominative, used for the subject
• их = accusative/genitive form, used here as the object

So:

• Они стоят на столе = They are standing on the table
• Их лучше не использовать = It is better not to use them

Since духи is grammatically plural, the pronoun referring back to it is also plural: их.

This is a very common Russian impersonal recommendation pattern.

лучше не использовать literally means something like:

better not to use
or
it is better not to use

Russian often leaves the subject unstated when English would say you in a general sense.

So:

• их лучше не использовать = it is better not to use them
• natural English: you'd better not use them or it's better not to use them

The sentence is giving general advice, so Russian does not need to state you explicitly.

Слишком много means too much or too many.

In this sentence it means in too great an amount:

их лучше не использовать слишком много = it's better not to use too much of them / not to use them in excessive amount

With something like perfume, English often prefers too much rather than too many, because we are talking about quantity or intensity, not countable items.

You can think of it as shortening a fuller idea such as:

не использовать слишком много духов = not to use too much perfume

It is understandable, but many native speakers would probably choose a slightly more standard concessive structure.

For example:

• Хотя духи и лёгкие, в закрытой комнате запах становится сильнее, поэтому в салоне лучше не использовать их слишком много.
• Пусть духи и лёгкие, но в закрытой комнате запах становится сильнее, поэтому в салоне их лучше не использовать слишком много.

Why? Because with пусть in this concessive meaning, Russian often likes an explicit contrast later, often with но.

So the original sentence is not wrong, but a learner should know that Хотя ... or Пусть ..., но ... may sound a bit more natural in many contexts.