There are 4 broad scenarios
(1) that there is someone who knows the replies but cannot serve
(2) there are others present who do not know how to serve or the responses
(3) there is someone who knows to serve but not the replies
(4) there is no one except the priest
For (1), (2) and (4) the celebrant moves the missal himself - without, however, observing the server’s customary genuflections. The cruets and lavabo bowl (practically, filled with water) are kept at the Epistle side of the altar. For the ablutions, there are two possibilities. The one that was favoured by the Italian rubricists was pouring the water and wine over the canonical fingers of the left hand, and then dipping the canonical fingers of the right hand into the chalice. It was also possible to pour water and wine on fingers of the right hand, after the left.
For (2) - (4), the priest recites the Confiteor once, omits “vobis/vos fratres” and says “Misereatur nostri” . As the missal indicates, for the Orate fratres, he says “de manibus meis”.
As mentioned earlier, if a woman was making the responses, it would have to be done from outside the altar rail. Several rubricists favor the opinion that a man who does not know how to serve should also make the responses from outside the sanctuary.