He was willing to marry Louisa if he had to, but only as a last resort - and who can blame him, he was in love with someone else. As to leaving when he did:

"He would gladly weaken, by any fair means, whatever feelings or speculations concerning him might exist" (ch 23)

It was all speculation concerning himself and Louisa. If his absence could ease that speculation ("Maybe we were wrong and he doesn't love her - he'd be hovering over her if he did") all the better. He saw that move as "fair means" not abdicating his responsibility. If Louisa et al still felt they were bound to each other when she'd recovered, he'd marry her.