Exactly right. Gaspard didn't force her to change her plans for Halamshiral, she made that choice all on her own. She had plenty of ways she could have reacted to a play that a couple of hundred people, at most, saw. From laughing it off to "Yeah, I bang a servant girl now and again. Who doesn't?" If she were as smart or adept at The Game as were supposed to believe, she'd have found a political solution.
Like, I don't know, one of the several political marriage offers on the table. If she'd responded positively to Remache's offer originally, instead of clinging to her elf body pillow, Gaspard would have lost one of his strongest supporters before he had a chance to put anything into motion.
I agree that Gaspard didn't force Celene's actions, he merely set the situation up so that she didn't have any good choices, much like Leliana did with the blood mages in The Exiled Prince. He baited a trap, and Celene fell into it. Interestingly, I doubt Celene would have reacted so strongly if Leliana hadn't made it clear that squashing the uprising was desirable to the Chantry as well. I doubt a skilled manipulator like Celene was unaware of what was being left unsaid. Celene was railroaded, but it wasn't by Gaspard.
As far as accepting marriage proposals, I don't know if Celene would ever be able to accept one. Remache had nothing to offer her other than his own land, which she held as empress anyway. The fact that Remache was equally willing to offer his daughter to Gaspard (who neither confirms nor denies Remache's expectations) shows that he's just trying to weasel into a better political position, much like Rendon Howe. I think Celene would have been better off accepting Gaspard's proposal, TBH. He had power and supporters of his own, but that wasn't what she wanted. She wanted to be the only one holding the reins. For a lover, Celene chose someone who had no political power of her own except that which Celene allowed her.