I think 'лишь только' is a colloquial expression, since it is a tautology, that is used for intensifying though. I did hear the expression in speech and it could be found out in poetry, but you must remember that it is not a literary expression.
I can imagine situations where either would fit
I don't think so; the senses 'as soon as' and 'only' are different enough to be told one from the other, but only based on context.
If you know such an ambiguous sentence, you may write it here and we'll try to examine it.