I believe Ronald is genuine.
Having replayed through MC's first two days with Stella, I think I'm more inclined to agree. The first scene at Ronald's home, he's very protective of her ("if you hurt her..."), thinks well of her ("she's a good girl"), and is almost apologetic of her quirks ("she's sometimes childlike"). Very much like a father, and maybe even one who wants to give a green light to MC to pursue Stella.
Next time he's over there, Stella teaches him, he gets her number, and he texts her later that evening. Whereupon she tells Ronald, "I got a text from a boy" (why she didn't just name MC at that time is unclear). Ronald's initial response sounds angry, but after she confirms it's MC, his later lines are "I'm so proud of you," "are you in love with him" (which, as she correctly notes, is entirely premature), and "keep talking to him, this is your chance." I think we can infer from this that (1) she has some concerns/frustrations/disappointments with boys, (2) she's shared those with Ronald, and (3) they both perceive MC texting Stella as indicating his romantic interest.
Still leaves a lot of questions, though; foremost in my mind are (1) how would she have had a lack of male interest, and (2) what's the best-case goal of the relationship? On the first, she's awfully cute, she's sweet, and she seems entirely likable. It's hard to imagine that she wouldn't have had male attention, unless she has just been sheltered enough to not have encountered boys/men (other than Ronald, of course) previously.
But the second is the bigger one IMO. Both she and Ronald are clearly thinking "relationship," not "casually date." Fine, but relationship leading toward what? I know I'm repeating myself, but according to the lore we're given in-game, slaves can't marry. So if she is actually a slave (and MC's seen the collar/chain), it seems the best case is she becomes MC's slave (rather like Cadence), not Ronald's.
But also on this play-through, I'm noticing Ronald's cough more. I wonder if this is headed toward "Ronald's health is much worse than he's letting on, and he's concerned about Stella's being taken care of when he's gone (which he expects to be soon)."