Well, Lope tends to be very strict in his conservative readings of text. I have trouble reading his grammars and parse trees, but *as far as I can figure it out*, the problem is that the question mark comes after the display, not have the sentence that sets up the display (which is a question all right). That is, he would insist on 'mi ken ala ken toki e ni? tenpo pini lili la mi open kama sona e toki pona.'
The theological ground here is that tp has no subordinate clauses (we skip over 'la', which is strictly coordinate) and so the second sentence is not part of the question, but an add-on, the referent of 'ni'. This isn't strictly a part of grammar (well, it is in Lope's system, which has a lot of punctuation rules built in), but of punctuation conventions. I think that the community would generally do things your way, but there have been a number of complex cases, where the display (what the 'ni' points to) are also long and complex, where disputes have arisen. Another way of putting this is that there is a dispute about just what is a sentence in tp. (I tend to set the 'ni' referents aside for separate analysis and then stick them back into the original sentence after the colon but preserving the original final punctuation. Lope would reject all of this out of hand, but it does cover most community usage pretty well.)