You can actually edit messages on slack mobile, just tap and hold on it, or tap it then click the ••• button. Your solution is pretty cool on one hand because it's built around how people already 'correct' their messages, but it also requires you to notice the mistake, so it doesn't completely solve the problem and still makes you re-read the message a few times.
What I go for is having spellcheck on and autocorrect off, so it can flag typos before I send the message.
Yes. I do this anyway, so if it auto replaced the word I spelled wrong that'd be perfect.
Also just a head's up - Your companies website doesnt work if you have Ublock. It only displays the header then nothing else works. Also it tried to prevent you from entering it in the first place.