It's difficult to imagine Calvin or any other paedobaptist making statements like that.
Charlie, at least with respect to what the statement says about baptism, I actually don't find it difficult to imagine at all. I'm not a student of Justin's theology, so I don't presume to speak authoritatively on his meaning - but I do believe strongly in infant baptism, and find the above not objectionable from that perspective. First, note the context in which Justin is speaking: he is not giving a comprehensive treatment or theology of baptism. He begins by stating, "I will also relate the manner in which we dedicated ourselves to God when we had been made new through Christ." No matter what he says from this point on, he is speaking within the context of people who are consciously consciously coming to the laver. Subsequent statements therefore should nott be read as pertaining to something outside this context. Likewise, Calvin begins the fifteenth chapter of Institutes IV by stating, "For it is his will that all who have believed, be baptised for the remission of sins." Such a statement, if taken for a strict definition (i.e., if we read Calvin as attempting to say more than he actually is), it would exclude infants, nor does he say anything in this portion of his work which speaks to baptism as pertaining to infants. But we know he does not maintain an antipaedobaptist position. As to the quotation you find questionable, about becoming children of choice and not of necessity by way of baptism, we find a similar type of statement in this chapter of Calvin:
Baptism serves as our confession before men, inasmuch as it is a mark by which we openly declare that we wish to be ranked among the people of God, by which we testify that we concur with all Christians in the worship of one God, and in one religion; by which, in short, we publicly assert our faith, so that not only do our hearts breathe, but our tongues also, and all the members of our body, in every way they can, proclaim the praise of God.
It's different language, but the idea is fundamentally the same: we wish to no longer belong to the earthly man/people, but we wish/will/choose to become the people/children of God. As a paedobaptist, I have no problem with such statements, but affirm them wholeheartedly.
Again, I cannot honestly say, "This is what Justin thinks about baptism;" But I
can say, it would take a whole lot more than a few quotes about believers' baptism for someone to even begin to make a claim about what he says; and I can also say that the quotes provided do not vex me as a paedobaptist.
(Sorry it's taken me a long time to respond; not much time these days)