Great AAR so far Tribulation.
I guess I'm not following your point about Austin, though. While Austin was supposed to have been personally against slavery, his public views on the subject weren't exactly consistent. Mostly, like your Houston, he was interested in expanding and civilizing Texas and figured that to do that, he would need educated, wealthy immigrants from the southern States. In other words, people who would want to bring slaves with them.
So whatever he may have felt personally, he was enough in favour of slavery to ask the Mexican government (pre-independence, obviously) to make Tejas an exception to that provision in the constitution of '24.