Here are two issues here.
- Whether members of the congregation may genuflect during Mass.
- Whether it is appropriate to reverence the Blessed Sacrament once someone has just received.
For the first issue, the General Instruction specifies particular genuflections of the priest and other ministers (No 274). While it is true that the laity participate, they do not do so as ministers. Thus the following instruction applies, “all who pass before the Most Blessed Sacrament genuflect, unless they are moving in procession.” (274)
Is returning the pew part of a procession? I do not think so. It is true that the faithful coming to receive Communion is a procession, “the communicants, who…approach in a procession.” (160) But I believe it would be incorrect to say that the return to the pew is a procession.
For the second issue, whether it is appropriate to reverence what has just been received, the answer is yes. In fact, the General Instruction prescribes it when receiving both species.
“160. When receiving Holy Communion, the communicant bows his or her head before the Sacrament as a gesture of reverence and receives the Body of the Lord from the minister. The consecrated host may be received either on the tongue or in the hand, at the discretion of each communicant. When Holy Communion is received under both kinds, the sign of reverence is also made before receiving the Precious Blood.”
As a side comment, Tabernacle should have a consecrated Host for the monstrance at benediction, therefore it would not be empty.