"If" and "Whether" - Is interrogative IF always replacable by WHETHER?

In short no. There are some cases where the word 'if' is not used as a conditional statement. The word 'if' might be used in an idiom or some other way that doesn't represent a conditional statement. In those cases whether cannot be replaced in its stead. Below is an example of how if is used in each case and shows when it can be replaced with the word 'whether'.

If comes from Old English. Below are the definitions and usages of if. I will interlace the solution within the solution within the definition below.

in case that; granting or supposing that; on condition that': Sing if you want to. Stay indoors if it rains. I'll go if you do.

For the above case yes you can replace whether with if

(2) even though: an enthusiastic if small audience.

in this case no. Placing whether in place of if does not constitute the meaning of 'even though'.

(3) whether: He asked if I knew Spanish.

Of course, in this case, it is replaceable. We can say He asked whether I knew Spanish.

(4) used to introduce an exclamatory phrase: If only Dad could see me now!

In the case of an exclamatory phrase whether does not fit or make sense. 'If only' is often used an exclamatory or an expression of emotion.

(5) when or whenever: If it was raining, we had to play inside. noun

In the case above 'when' or 'whenever' can be replaced with 'if'. Grammatically a rhetorical statement is stated followed by a comma and then a proposed action. E.g. If I was a millionaire, I'd quit my job. So, in this case, no you can't replace it.

(6) a supposition; uncertain possibility: The future is full of ifs.

In this case, the word 'ifs' represents possibilities or uncertainties. So the answer to this one would be no you can't in this case.

(7) a condition, requirement, or stipulation: There are too many ifs in his agreement.

The word 'ifs' also can represent conditions to be fulfilled. So the answer to this one would be no you can't in this case.

(8) ifs, ands, or buts, reservations, restrictions, or excuses:I want that job finished today, and no ifs, ands, or buts.

In this case 'ifs' is used in an idiom stating in short 'no question about it'. It might even be considered an exclamatory statement basically said with authority. The supplemented statement might look like this if updated for someone who doesn't understand the idiom.

I want that job finished today, and there will be no excuses and you will finish it regardless.
or I want that job finished today! Implying you will not make any excuses be

So the answer to this one would be no you can't in this case. Whenever 'if' or 'ifs' is used in an idiom you can't replace it with 'whether'.

• There are a few verbs where I think "<verb> <interrogative clause>" in general is colloquial-but-acceptable, but where I think *"<verb> whether […]" in particular is ungrammatical; for example, consider "went to look if […]" vs. *"went to look whether […]", or "I can't think if I've […]" vs. *"I can't think whether I've […]".

  • In part this might be because whether is a bit more formal than if, so it doesn't work in these colloquialisms because the registers don't match; but I don't think that's a complete explanation, because I don't think the registers are so mismatched as to account for why the result sounds so ungrammatical (at least to me). (I'd welcome your thoughts on this.)

• This is probably cheating, but I don't think if can be replaced with whether in an echo question:

  "I'm wondering if she, uh . . ." He trailed off.
  *"Whether she what?"

  (Note that this isn't completely trivial, since echo questions do allow some kinds of substitutions; for example, if the first speaker had said "Hannah" instead of "she", it would still be fine for the second speaker to substitute "she". And I think that echo questions can usually replace the zero complementizer with that and vice versa. But even so, I describe this as "probably cheating" because it's probably more a fact about echo questions than a fact about if and whether.)

• You yourself pointed out in a comment above that something like "ask [if [whether …] …]" is clearly intelligible and grammatical, whereas something like "ask [whether [whether …] …]" is, um, not.

• This is obviously cheating, but hey: colloquial/dialectal if'n can never be replaced with whether'n. ;-)

This time I have read your post more carefully. I am guessing that the answer is "no". Whether can be used wherever interrogative if can. Conditional if has restricted distribution (lower type-wise frequency), but is more frequent token-wise. You see this a lot when looking at two words with similar meaning.

I have plotted below the top 50 hits for verb+if (red) (discarding instances of conditional if) and verb+whether (blue) from COCA to give you an idea.