Spelling is not exactly the same thing as ‘pronunciation guide’, and the Basic Roman Spelling (BR) is not meant as a phonetic system. However, you have been so persistent here on the possibility of using BR as a phonetic alphabet (and I do appreciate your criticism) that you made me consider this problem too. As BR is very close to full phonemicity (is there such a word?), I designed one possible version of a phonetic alphabet for English based on BR, with very few modifications, let’s call it Roman Phonetic Alphabet for English (RPA). Unlike BR it provides for the one-to-one correspondence between phonemes and graphemes at word level. Its principal difference from IPA is that it uses Roman letters alone, plus some stress marks. You may see the result posted in my Re-Romanization website, the entry
‘Roman Phonetic Alphabet for English’
You are putting some pretty difficult questions, and let me say in the first place that I am not the proponent of spelling reform that ought to give the answers. There is a good summary of the arguments in favour and against in the Wikipedia article
‘English spelling reform’
Will spelling reform have an effect on spoken language? Phonemic spelling is supposed to reflect spoken language as it is, not influence it. However, my guess is that there might be such an impact. A relevant recent example of similar influence is seen in the case of Bulgarian language. Namely, while Bulgarian is written in Cyrillic there is an extensive mass practice of writing Romanized Bulgarian in e-mails and SMS messages. As a result, people now make new kinds of writing errors in Cyrillic that never existed before; but tomorrow’s rules are today’s errors.
How to spell ‘can’ in BR? Well, I would spell it ‘kan’; [kIn] (if the Merriam-Webster transcription is meant) would be spelled ‘kayn’. However, I don’t have the answer to the real question here: If we have the phonemic spelling available, should one write exactly as one speaks (local dialect) or write in some mainstream version of English s.a. Received Pronunciation, American English, Australian English etc? Me, I like the traditional English spelling, which I consider hyerogliphic – one hyerogliph (albeit constructed from letters) for each English word; yet spelling reform advocates may have some point as far as the education of children and non-native speakers is concerned (not the reeducation of people accustomed to traditional spelling).