To me, the second substitution doesn't sound natural. In your first sentence, "whatsoever" is an adjective, or maybe a determiner, that attaches to "idea". That substitution works. In the second example, there is no noun that "whatsoever " can attach to. While some dictionaries show "whatsoever" as an adverb, some don't, and it doesn't sound right to me.
As to the difficulty in pronunciation, can you pronounce the word "tall"? If you can, you might try practicing saying "a tall man" until you can say it easily, and then drop off the "man", and then gradually saying "a tall" more smoothly as one word.