Grammatically speaking, the answer is no.

As far as I know noun clauses of if cannot serve as the object following a preposition. Nor does “as to if…” nor in if…” in a sentence is valid.
All noun clauses directed by if can serve as the direct object of a transitive verb, so it is ok to say

I’m curious to know if you’re still interested.

Where “if you’re still interested” serves as the object of know.

Is it possible to replace by whether in your original sentence, because noun clauses led by whether can be used almost the same as is an ordinary noun: it can serve as the subject, the noun predicate, or the object of both verbs and prepositions. You can say I’m

curious as to whether you are still interested.

Which alternative is grammatically correct and sounds acceptable to me while the previous one is not so good?

Grammatically speaking, the answer is no.

As far as I know noun clauses of if cannot serve as the object following a preposition. Nor does “as to if…” nor in if…” in a sentence is valid.
All noun clauses directed by if can serve as the direct object of a transitive verb, so it is ok to say

I’m curious to know if you’re still interested.

Where “if you’re still interested” serves as the object of know.

Is it possible to replace by whether in your original sentence, because noun clauses led by whether can be used almost the same as is an ordinary noun: it can serve as the subject, the noun predicate, or the object of both verbs and prepositions. You can say I’m

curious as to whether you are still interested.

Which alternative is grammatically correct and sounds acceptable to me while the previous one is not so good?

Grammatically speaking, the answer is no.

As far as I know noun clauses of if cannot serve as the object following a preposition. Nor does “as to if…” nor in if…” in a sentence is valid.
All noun clauses directed by if can serve as the direct object of a transitive verb, so it is ok to say

I’m curious to know if you’re still interested.

Where “if you’re still interested” serves as the object of know.

Is it possible to replace by whether in your original sentence, because noun clauses led by whether can be used almost the same as is an ordinary noun: it can serve as the subject, the noun predicate, or the object of both verbs and prepositions. You can say I’m

curious as to whether you are still interested.

Which alternative is grammatically correct and sounds acceptable to me while the previous one is not so good?

Grammatically speaking, the answer is no.

As far as I know noun clauses of if cannot serve as the object following a preposition. Nor does “as to if…” nor in if…” in a sentence is valid.
All noun clauses directed by if can serve as the direct object of a transitive verb, so it is ok to say

I’m curious to know if you’re still interested.

Where “if you’re still interested” serves as the object of know.

Is it possible to replace by whether in your original sentence, because noun clauses led by whether can be used almost the same as is an ordinary noun: it can serve as the subject, the noun predicate, or the object of both verbs and prepositions. You can say I’m

curious as to whether you are still interested.

Which alternative is grammatically correct and sounds acceptable to me while the previous one is not so good?

Grammatically speaking, the answer is no.

As far as I know noun clauses of if cannot serve as the object following a preposition. Nor does “as to if…” nor in if…” in a sentence is valid.
All noun clauses directed by if can serve as the direct object of a transitive verb, so it is ok to say

I’m curious to know if you’re still interested.

Where “if you’re still interested” serves as the object of know.

Is it possible to replace by whether in your original sentence, because noun clauses led by whether can be used almost the same as is an ordinary noun: it can serve as the subject, the noun predicate, or the object of both verbs and prepositions. You can say I’m

curious as to whether you are still interested.

Which alternative is grammatically correct and sounds acceptable to me while the previous one is not so good?

Grammatically speaking, the answer is no.

As far as I know noun clauses of if cannot serve as the object following a preposition. Nor does “as to if…” nor in if…” in a sentence is valid.
All noun clauses directed by if can serve as the direct object of a transitive verb, so it is ok to say

I’m curious to know if you’re still interested.

Where “if you’re still interested” serves as the object of know.

Is it possible to replace by whether in your original sentence, because noun clauses led by whether can be used almost the same as is an ordinary noun: it can serve as the subject, the noun predicate, or the object of both verbs and prepositions. You can say I’m

curious as to whether you are still interested.

Which alternative is grammatically correct and sounds acceptable to me while the previous one is not so good?