I don't think it is in Merck's interest to be buying shares on...

I don't think it is in Merck's interest to be buying shares on market as they will not be able to vote them in the Scheme of Arrangement vote.

Quote from Minter Ellison in my earlier comment
"A pre-bid stake may be a disadvantage under a scheme because those shares will not be voted in the same class as other target securityholders to approve the scheme, therefore enlarging the effective vote of all other target shareholders on the scheme resolution;"

That is, they would be reducing the pool of shareholders who are happy with the bid and sell out early in the bid process to Merck (that is the yes voters), leaving a higher percentage who do not want to see the vote succeed (the no voters). Hence making it easier to knock the proposal back.