Was the matter of whether the Christians who had renounced their faith under torture should be accepted back considered in the First Council?

Short answer

In the pre-Nicaean era, lapsi referred to those who renounced their faith under persecution by Roman authorities. During the AD 250 Decian persecution most who lapsed were not tortured, but were required to sacrifice in a court proceeding. See the wikipedia article Lapsi for the 5 ways of how they lapsed. Valerian persecution was harsher, ordering bishops to death and senators to lose their title and properties if they would not apostasize. After about 40 year respite, the most severe persecution happened under Emperor Diocletian in AD 303-306 which included razing churches, burning scriptures, imprisonment of all bishops and priests, and involving torture if they didn't apostasize by offering sacrifice willingly.

With most who didn't lapse died under torture / execution, it makes sense that the AD 325 council didn't specifically use the category of torture as a criteria for accepting back those who lapsed, but instead decreed how to variously deal with different groups of people who lapsed. About the schismatic groups, they are NOT those who lapsed but in fact those who refused reconciliation with the lapsed.

Here is a summary of how the council dealt with the lapsed:

• Canon 8: Allow the separatist clergy & bishops (started AD 250) called the Cathari or the Novationists, who refused forgiveness to those who lapsed during persecution, to re-join the apostolic church provided they accepted those who lapsed
• Canon 10: To depose any in the clerical rank who has lapsed
• Canon 11: Penance for those who repent after having "fallen without compulsion, without the spoiling of their property, without danger or the like" (many were communicants)
• Canon 13: Lenience for the lapsed who are dying
• Canon 14: Penance for lapsed catechumens
• Synodal Letter to the Church of Alexandria: Disciplined the separatist leader (Meletius) of the "Church of Martyrs, a group of fanatical confessors, who thought that their privileges as sufferers for the faith were not sufficiently recognized by the Church", thus indirectly giving leniency to the lapsed. See also the 1912 Catholic Encyclopedia article on Meletius of Lycopolis.

Reading between the lines, since they dealt mercifully with different groups of people who lapsed, including those who lapsed "without compulsion" (see Canon 11), they must have accepted back those who lapsed under torture, possibly with lesser penance.

Longer answer

The authoritative documents from the First Council of Nicaea (325 AD) which consists of:

• The Profession of Faith
• Canons
• Letter of the Synod in Nicaea to the Egyptians

can be read below:

• here (source: Nicene and Post-Nicene Fathers: Second Series Vol 14, ed. Philip Schaff) or
• here (source: Decrees of the Ecumenical Councils ed. Norman P. Tanner)
• here (only the Canons portion, but has helpful commentaries)

Another resource is from the 1983 book The First Seven Ecumenical Councils (325-787): Their History and Theology by Leo Donald Davis, SJ, who wrote the book to fill a lacuna of integrated history + theology of the councils when he taught his students at Weston School of Theology. He said that the official minutes of the sessions have not survived. What follows are his explanation of the canons.

...

"Six other canons dealing with the dignity of the clergy:

...

Canon 10 enjoined the deposition from clerical ranks of anyone who had denied his faith, whether he had been ordained in ignorance of this fact or not.

...

"Four other canons dealt with the reconciliation of those who had lapsed from the faith during the recent persecutions and with their public penance.

Canon 11 provided that those who fell away from the faith without having been threatened must repent and then spend two years among those who could only hear, not participate in the liturgy, seven years among those required to kneel before their fellow Christians on Sundays to beg forgiveness — two classes two classes which with the catechumens were compelled to leave the liturgy before the beginning of the Canon — and for an additional two years continue to remain during the whole liturgy but without receiving the Eucharist.

...

According to canon 13, the lapsed who were dying were allowed to receive the Eucharist, but if they recovered, they were to attend the liturgy only.

In canon 14, catechumens who had lapsed were ordered to remain among the hearers of the liturgy for three years and then take their places as catechumens once again. Severe as these measures were, they were more moderate than any previous synodal decrees, especially in providing that the lapsed when dying could receive the Eucharist.

"Two canons dealt with the more difficult problem of the readmission to the Church of schismatics and heretics.

...

Since the Novatianists, who broke with the Church over the question of penance from 251 on, differed only in discipline and not in doctrine, they were treated with great moderation. After having received the imposition of hands and professed in writing that they would follow the decrees of the Church and in particular that they would communicate with the twice married and the reconciled lapsed (two classes of sinners which as Novatianists they shunned), they might be restored to the clergy at the rank they held as Novatianists. However, in places where there was a Catholic bishop, the reconciled Novatianist bishop was to be made only a chorepiscopus, a rural auxiliary bishop, so that no city would have two bishops.

...

"In a separate declaration,

the Council dealt with the Meletian schism in Egypt which grew out of the attempt of the priest Meletius to exercise episcopal functions while the bishop Peter of Alexandria was imprisoned during the persecution of Diocletian. Since Meletius too had been imprisoned in the quarries of Egypt, he used his prestige as a confessor of the faith to put himself at the head of the Church of Martyrs, a group of fanatical confessors, who thought that their privileges as sufferers for the faith were not sufficiently recognized by the Church. By 325 Meletius’ church numbered some twenty-eight bishops. Overriding Alexander of Alexandria’s apprehensions, the Council promised the Meletians that their ordinations would be recognized when they returned to the Church, on condition that their bishops cease exercising their functions in favor of those consecrated by Alexander. Meletius himself was ordered to withdraw to Lycopolis, content himself with his title of bishop and discontinue further ordinations.