I grant you, there could have been something of a hidden symbolic meaning in a black horse for Angel, but Alec doesn't come and save Tess at that grey farm, on the contrary, as the reference to Paradise Lost denotes later when she is working on the field (almost in Hell, so to say), and Alec turns up, he is not a good influence. His reformation is a shallow one, as is apparent from how quickly he changes back to his old self. All because of Tess, as he says. Yeah right. Real rakes do not easily reform. As the PL reference lets shine through, he is there not to merely give her a little nudge into the abbyss (the rape or at least sexual encounter they had in the Chase), but to really pull her into its deepest regions (the kind of hovering spirit of the underworld she has become when Angel sees her back). From a soiled woman she can fall only deeper by becoming a mistress, after that discarded because her youth and beauty woudl fade, after which she would become a prostitute. The Victorian mind was pretty straight about that.
The sad thing in Naturalist novels is that decisions indeed don't matter. Whatever decision a character makes, his fate will be the same. I.e. Tess would have been hanged for murder (or at least died early), maybe not because she killed Alec, but because of something else. She would have been raped anyway, whether her father had known about the d'Urberviles or not.
That is a total misconception. Angel's reaction is logical and unavoidable. Hardy obviously thought that was unjust, on a human level, but still it was logical, as the priest's reaction is logical when he refuses to burry Sorrow in a consecrated grave. Unjust definitely, but unavoidable.
Hence why Tess's mother said not to tell Angel. He would not have noticed (there were ample ways to get around it, pig's blood being one). If she tells him, though, she is no longer a virgin (a matter of course), her character is not as pure as he thought it was (the only thing a woman really had to recommend her) and who is to say she would not do it again, will he be sure his children are really his?
To modern people this is unjust and cruel, but to Victorians soiled women were prostitutes, they were dirty things never to be seen. Indeed Jude features a family with several children, with two parents who love each other but can't marry. SPOILER They are eventually compelled to throw in the twoel, because that fact follows them everywhere. SPOILER OVER I think there would have been very few men who would have taken a wife who wasn't a virgin and hadn't been married before.
Tess, in her naivety that Angel loves her and has a wider look on things than a Victorian one, tells him because she wants to be straight and because he confesses to her he had a fling. She feels stronger because of that. That is her big mistake (and what her mother warned her for). Men were allowed to do this, women were not. Indeed, the only ground for divorce was if the wife cheated, and then the other man was a co-respondent. To get rid of a man, he needed to be violent and a serial womaniser with several mistresses on the go at one time, combined. And even then he mostly got the children. The point being that women were supposed to be pure and homely.
To me he is unlikable, because Angel is basically two-faced. Oh he is so aloof of everything, he's so broad-minded etc. and then when push comes to shove (his wife is not a virgin), he turns into this Victorian thing his father is, the very thing he despises. The only thing what's missing is really the fact that he would have asked for a dowry. Eventually he turns likable again, but it is too late. Still, the very fact that he is more Victorian than he wishes, that he is shaped by his father (although he wishes this were not true) and that he cannot consciously get over this, is also a sad Naturalist tragedy: whatever he does, his thoughts are unavoidable.