Are the phrases 'to all intents and purposes' and 'to all extents and purposes' interchangeable?

"All" needs to modify a plural noun, so "intents and purposes" would be correct.

"For all intents and purposes" is the correct phrase according to American usage. The British version of the idiom is "To all intents and purposes." (See the Related link.)

The correct saying is "all intents and purposes".

'to all extent and purposes' means that it is correct for every possible scenario, so from what ever angle you were to look at it it would be true.

Neither of the phrases in the question is correct. The correct wording, in American English, is "for all intents and purposes." In British English, however, the wording of the selfsame idiom is "to...