Yes. If you decide to give horse away, there is chance that horse trader visits your settlement and asks if you want to buy a horse
Regarding Vincent is there way to slow the relationship gain back down. I bought an axe and three ropes to make a bow and end with 20 relationship points seems a little rushed to me since I was only 15 days in.
Before it took longer about 40-45 days to get to that point, which makes more sense to me anyway especially if he is going to entrust his daughter to you.
I usually like to get the guest house, greenhouse and hot shower taken care of before having to deal with Blair. And yes I tend to start over with every new version to see if I can do better.
Also one other thing I noticed you can turn slaves into guests before you have the guest house built.