Glad you like it, more to come :D
Regarding the collars, the game as it is at the moment lets you collar of the girls so far, but the events that let you collar them are basically final route selection. The currently collarable girls are Sasha and Riley, but unless you decided not to punish one of them, I'm unsure how you only got to collar one of them. Did you mean you only decided to collar one, or that you only got the chance to collar one?