Break the argument down here: Defensor Fidei agrees that Communion in the hand is less good than Communion on the tongue (why?). But his argument seems to go like this: It was allowed in the early Church (a question of fact which has yet to be proved insofar as "Communion in the hand" is understood to be the regular way of doing things) so, therefore, it is not a sin."
IT is not a question of fact. Taking the information we have provided by those who have studied the Liturgy, it is shown that CIH not just existed up through the 7th century, it is a logical use when you consider things like practicality leavened bread and the development of the rites.
Obviously, there is nothing inherently sinful about touching the Host, or else one could never touch it for any reason, even so that good may come from it, such as rescuing it from Satanists or what not. But it is apparent from the recent past that Communion in the hand leads to desecration, to disbelief in the Real Presence, to a blurring of the lines between the ordained and royal priesthoods, etc. Knowing this is so -- knowing that Communion in the hand harms the proper understanding of the Faith and to desecration (dropped particles, etc.) -- to needlessly handle the Sacrament is sacrilegious, which is a sin.
I disagree with your opinion (in bold). Reading the development of the Liturgy it is evident that disbelief in the RP was not the issue. The issues of concern had to do with people abusing the sacrament by not eating it.