twitter's leverage is limited by structural constraints in maintaining a good developer ecosystem.

Which they've demonstrated that they are willing to test the limits on multiple occasions recently (ubersocial, "no new clients" post, etc).

Twitter have explicitly said they don't see a lot of value in third-party client software, so that doesn't seem likely to constrain them here.

I'd think the law limits their leverage, too.

I don't think twitter can threaten, directly or indirectly, to cripple tweetdeck in any way if the sale doesn't go through or if they sell to another party.