Is "please let me know if otherwise" grammatical?

Solution 1:

I often come across if otherwise, if not, if opposed, and the like in business communications as a shorthand. I would avoid them if possible.

Otherwise means differently or alternatively, so if otherwise posits an alternative scenario. The trick, of course, is to be mindful of which scenario is being referred to, especially when context is limited. To the original question, if you wanted to use if otherwise to see if the boss agrees with your approach, you would need to write something like

I am planning to do X and Y. I think that is the approach you prefer; please let me know if otherwise.

As originally written, you ask

I am planning to do X and Y; please let me know if I am not planning to do X and Y

which strains logic.

Using otherwise in this way can be quite vague, as there are a number of alternative scenarios which could equally apply:

… if someone else is planning to do X and Y [instead of me]

… if someone else is already doing X and Y [instead of planning]

… if someone else is planning to do A and B [instead of X and Y]

It is better to write explicitly:

I am planning to do X and Y. Please let me know if you prefer a different approach.