Yup, sorry, I sometimes do that (write, then carefully read the original post).
If we step away from toki pona, [which is a system with a largely absent inventor and a bunch of transient, disorganized fans], and talk about conlang creation in general, I agree with you on capitalization and using the general patterns for using the latin alphabet, punctuation etc. If I were to write my own conlang, my audience would be other people in Arlington, Virginia and I would make sure it was writable with ordinary a to z (no diacritics) and used punctuation more or less like English unless there was something seriously compelling about inventing a new punctuation system, a letter with a diacritic, or some unexpected convention for capitalization. Prestige scripts are a different story-- those are for making stuff that looks nice on the wall, not as a communication system that people will actually use, on forums or in real life.
Since the inventor is largely absent and the fans are transient and disorganized, ideas to change existing conventions don't catch on, or do so slowly. We are in the same state that Japanese is with their god awful complex writing system. People are sticking with the writing system because it's an established convention. I think to change the language now, someone would have to use toki pona & contribute enough to the corpus that people start to imitate you. For example I, on this forum, for a while so consistently misspelled tenpo as tempo that some people began copying me. I didn't do that on purpose. I try to spell the word correctly now.
If tp lacked a period, then we'd have no written sentence dividers. The capital is redundant for starting a sentence. In the spirit of doing with less, it makes sense to give up the initial capital. We could maybe give up the capital for proper modifiers because they are recognizable from not being on the list of 125 or so words. ma mewika is just as clear. But the edge case would be names like Mary : jan meli vs jan Meli. Verbally, you'd get no hints that the 2nd was a name and the former meant "woman" or "the woman's person."
For spoken toki pona, we don't have any good guidance yet. The fans of toki pona are from all over the place-- Finland, Russia, Spain, Brazil, Japan, and those languages don't have the same conventions for intonation, stress or volume (or pauses between words) to indicate end of sentence end, or questions. So we really don't know what's going to happen in spoken toki pona-- it's attractive to think that rising tone at the end will mean question and falling means end of sentence, but in Finnish (according to someone in my book club) rising tone means you're angry and I don't know what falling tone means in Finnish.
At the moment in written tp, questions aren't a problem because we got the question mark, sentences aren't a problem because we have the period.