Are the persons nominated to be created cardinals eligible to participate in the conclave before the consistory?
The short answer is no.
They are not Cardinals at the time of the death of the Supreme Pontiff of the Roman Church.
The next pope is not under any obligation to continue with the consistory the these nominations.
Remember that they are not cardinals. And as such they have no ecclesiastical privileges to that office. They could possibly be invited to participate in the cardinals’ discussions on the election of the new pope.
At the death of a pope everything is put on hold and the next pope is free to decide whether to go forward with anything not finished by his predecessor. For example, when Pope Francis became the Supreme Pontiff of the Roman Catholic Church he choose to continue with World Youth Day which had been put on hold when Pope Benedict XVI resigned.
It next pope thus would decide whether to go ahead with these nominations or not. Remember that cardinals are chosen by the pope himself. Whether he seeks help in this is an open question. The next pope may choose otherwise, but probably would not.
At the death of a pope. Cardinals in Pectore cease to exist!
As you said the following, it would not be a hinderance in his possibly being elected pope. The Cardinal-Electors know what is going on in ecclesiastical circles....
I am looking pretty much on the prefect-elect of the Dicastery for the Doctrine of the Faith, Víctor Manuel Fernández who may be elected pope in the next conclave if only he is properly made cardinal before that conclave.
The Cardinal-Electors could still vote for one of these men to become the next pope even if they are simply Cardinal-Designates!
The following article may be of interest: