I think it is wrong for the deacon to participate in prominent role as a reader in a Mass where he is not participating as a deacon. The 1963 Vatican II document, The Constitution on the Liturgy, Sacrosanctum Concilium has:
“28. In liturgical celebrations each one, minister or layperson, who has an office to perform, should do all of, but only, those parts which pertain to that office by the nature of the rite and the principles of liturgy.”
The General Instruction of the Roman Missal (GIRM) has in n. 171, it has that the Deacon:
“f) carries out the duties of other ministers himself, if necessary, when none of them is present.”
Also from the GIRM:
“59. The function of proclaiming the readings is by tradition not presidential but ministerial. Therefore the readings are to be read by a reader, but the Gospel by the Deacon or, in his absence, by another Priest. If, however, a Deacon or another Priest is not present, the Priest Celebrant himself should read the Gospel, and moreover, if no other suitable reader is present, the Priest Celebrant should also proclaim the other readings as well.”
“99. The lector is instituted to proclaim the readings from Sacred Scripture, with the exception of the Gospel. He may also announce the intentions for the Universal Prayer and, in the absence of a psalmist, recite the Psalm between the readings.
In the celebration of the Eucharist, the lector has his own proper function (cf. nos. 194-198), which he himself must carry out.”
“101. In the absence of an instituted lector, other lay people may be deputed to proclaim the readings from Sacred Scripture, people who are truly suited to carrying out this function and carefully prepared, so that by their hearing the readings from the sacred texts the faithful may conceive in their hearts a sweet and living affection for Sacred Scripture.”
“107. Liturgical functions that are not proper to the Priest or the Deacon and are mentioned above (nos. 100-106) may even be entrusted by means of a liturgical blessing or a temporary deputation to suitable lay persons chosen by the pastor or rector of the church.”
Continued in next post …