Your point number 1 and 3 aren't true for most people. -nt doesn't trigger pre-fortis clipping, in fact, the opposite happens in some varieties of English (i.e. the preceding vowel lengthens). Can is usually unstressed and is pronounced with a schwa as you say, but when it's stressed, it has /æ/ in AmE. I've never heard it with the BIT vowel (/ɪ/ -- usually referred to as KIT). Also, we can't say that can't is always stressed, because it's definitely unstressed in some contexts.
Transcriptions of can't: Even the pronunciation of a single word varies from person to person, so I can't represent the pronunciation of every speaker of American English, but here's my best shot:
- it's also possible though that the [k] might become a bit fronted3 in anticipation of the following front vowel, in which case [k̟ʰ-]
it can be one of the above in AmE
For can't ask, I heard something along the lines of [ˈkʰæ̃ɾ̃æsk] (or perhaps [ˈkʰæ̃ːɾ̃æsk]) from most speakers on Youglish.
[ɾ̃] is called a nasalised flap. [ɾ] is called a flap and appears in words like better, water, bitter (between a stressed and an unstressed vowel) in AmE. Nasalised flap, on the other hand, occurs in words such as win.ter, coun.ter, in.ternet. When a stressed syllable ends in an n and another (unstressed) syllable starts with a t, the -nt- usually becomes [ɾ̃] in some (most?) American accents. You can listen to [ˈwɪntɚ ˈwɪnɚ ˈwɪɾ̃ɚ] here at Wikipedia. The first one is winter with a [t], the second one is winner and the third one is winter with a nasalised flap.
Here's what Trask's Historical Linguistics says about the nasalisation of the vowel preceding a nasal:
Many English-speakers, particularly in North America, have conspicuous nasalization of vowels before a nasal consonant, in words like can’t, don’t and punt, and it takes only a slight delay in making the alveolar closure for the [n] to disappear altogether. Hence many Americans pronounce these words as [kæ̃t], [dõũt] and [pʌ̃t] with the nasalization of the vowel solely responsible for distinguishing these words from cat [kæt], dote [dout] and putt [pʌt].
Also from this answer on Linguistics SE:
When flapping applies to /t/ in "entertain", "ninety", /n/ actually deletes and the preceding vowel becomes nasalized, so you have [ɛ̃ɾ̃ɹ̩ˈtʰɛjn, ˈnãj̃ɾ̃ɪj]
So I would transcribe can't ask as [ˈkʰæ̃ɾ̃æsk].
- The superscript h [ʰ] represents aspiration; the puff of air that accompanies some consonants such as p, t, k when they're at the start of a stressed syllable.
- [t̚] is an unreleased 't', meaning there's no audible release.
- ‘An advanced or fronted sound is one that is pronounced farther to the front of the vocal tract than some reference point’ (Wikipedia)