This may be a more linguistic than ELL answer, but here goes:
(1) The stops t, p, k, when syllable-final, undergo glottal reinforcement in English. This minor glottal occlusion does not wholly impede the airstream. So, when the stop is released, the remainder of the air is released too. This is reminiscent of an unvoiced schwa, which accounts for what you hear after the t in cat. In terms of IPA transcription, one tends not to write the fine, automatic phonetic detail, and, so, for English, one marks only the preglottalization, as in [kʰæˀt].
(2) Crosslinguistically, the behaviour of syllable-final t, p, k varies. In Kiowa, for instance, as described by JP Harrington, t and p undergo complete glottal closure and are unreleased. In German, they are lightly post-aspirated. There are, however, only a certain number of perceptually distinct things you can do with your articulatory tract. So, my guess is, there'll be other languages that behave as English does.
The /n/ in IPA is never a mark of nasalization of the vowel preceding it. It always means your tongue is touching the tooth ridge and air passes through the nose. Therefore it is a nasal consonant. Vowels before a nasal consonant are technically nasal through assimilation, but in English, unlike some other languages, there is no phonemic distinction between nasal and oral vowels. Therefore you should not worry about it.
Both "dune" and "sun" end with the same sound, the only difference is in the vowel. The UK english pronunciation of "dune" differs slightly from the US: /djuːn/ vs. /duːn/. The IPA for english words does not mark nasalized phonemes in diphthongs because, again, there is no phonemic distinction between nasal and oral vowels in English. Perhaps you perceive a difference in the final "n" sound because the vowel in "dune" is a long vowel? Because of assimilation, you think the "n" sound is different: the only difference is in the duration of time air passes through the nose, the position of the tongue should be (more or less) identical.
The degree of nasalization of the diphthongs in "find" and "saint" will vary from person to person, and personally I hardly nasalize them at all.
Best Answer
Words like bot and bought originally had different vowel sounds, but for some North American speakers, these vowels are now merged to a single vowel phoneme. This merger is called the "cot-caught merger"; there is a previous question about it: Why are many TV personalities beginning to pronounce "daughter" as "dotter"?
For speakers with or without the merger, there are a variety of ways that these vowels can be realized phonetically. The vowel in bot (symbolized in the CMU pronunciation dictionary as AA) is most commonly transcribed in the International Phonetic Alphabet as [ɑ], but speakers with the "Northern Cities Vowel Shift" use a quality more like IPA [ä] instead. The vowel in bought (symbolized in the CMU pronunciation dictionary as AO) is most commonly transcribed in IPA as [ɔ], but some speakers use a quality closer to [ɒ], and the linked Wikipedia article says that speakers who pronounce bot with [ä] may pronounce bought with [ɑ].
The remarks in the previous paragraph apply to North American English specifically. Outside of North America, bot and bought are generally pronounced with two different back rounded vowels, the second distinguished from the first by a combination of greater length and greater closeness (a.k.a. greater vowel "height"). So a common convention for transcribing an "R.P." English accent is [ɒ] for bot and [ɔː] for bought; an alternative transcription that might better represent the usual modern values in England would be [ɔ] for bot and [oː] for bought.
You seem to be an American English speaker with the cot-caught merger. In that case, there isn't really a single vowel in your accent that is exactly equivalent to the "AO" phoneme, because as explained above, the AO phoneme has different phonetic values in different accents. The vowel that you use in words like orange and port might be similar, but that is not certain. The reason the CMU dictionary transcribes orange and port with "AO" is because some American English speakers with a distinction between the vowels in bot and bought find that their pronunciations of bought, orange, and port sound like they contain the same vowel phoneme. However, this isn't the case for all speakers of American English. Some speakers instead pronounce orange with the same vowel as bot or tar. Other speakers find that orange is best described as containing an allophone of the "long o" phoneme found in words like goat. (For example, tchrist, a fellow member of this site who distinguishes an /ɑ/ phoneme from an /ɔ/ phoneme, has stated that he finds the vowel in words like Tory seems to have neither of these two phonemes, but instead the third phoneme /o/).