Well, this is the first that I have read these letters, and I will try to do so in detail.
On a quick perusal, the following strikes me:
1. Orthodox Christians familiar with our canons are completely familiar with "epitimia" -- or penances assigned to someone by a father confessor. These are of the most ancient provenance in the Church, and generally involve (according to the canons) periods of be restricted from receiving the Mysteries.
2. Epitimia can be shortened at the discretion of a bishop or his designee, if felt in the best spiritual interest of the penitant.
3. St. Cyprian appears not to be very fond of these letters that are floating around -- he seems torn between wanting to honor and respect the martyrs (or more precisely, those awaiting martyrdom for the faith who are supposedly interceding on behalf of others,) and feeling that strictness should be applied.
4. We have been discussing purgatorial indulgences -- I see no evidence in these letters that St. Cyprian is discussing anything but the matter of those wishing to return to communion earlier than canonical strictness allows. Are you meaning to imply that those indulgences that we read about in pre Vatican II materials weren't for purgatory, but were rather to allow people to return to receiving Communion a few years early, or are you finding something in these letters of St. Cyprian about the afterlife that I'm missing?
5. St. Cyprian seems to make clear that those making decisions on these things need to examine carefully each situation, to make sure that the wise thing is being done for the good of the soul of the ones in question. He praises those who want to return to communion as wanting something good, but he also cautions that allowing such a return too quickly on the basis of these "letters from the (soon to be) martyrs" can bring a soul to ruin. His approach strikes me as a quite familiar "case by case" one that any Orthodox Christian with a spiritual father instantly recognizes. This is a far cry from a blanket indulgence granted for a set prayer or work.
6. St. Cyprian seems to be trying to put some order on these petitions by making the martyrs personally designate the specific person on whose behalf they are appealing, and by asking that granting the wishes be generally restricted to those who are close to fulfilling their canonically appointed period of not receiving communion or who are in danger of death. Again, this is a pretty case by case approach, and one that gives people the benefit of the doubt, so to speak.
7. Again, overall, this is not a very positive portrayal, and would appear to be a practice that didn't take root and become part of the universal Christian phronema. It certainly doesn't have anything to do with remitting temporal punishments in the afterlife.
An indulgence is what it is, and has no necessary connection to purgatory. Should I so wish, I could go about collecting indulgences soley for my own spiritual benefit, without any heed towards te Holy Souls.
What an indulgence is, specifically, is a remission of all (plenary) or a set amount (partial) of the canonical penance due a sin, in view of the excess merits of Christ and the Saints. For example, say a saint in their lifetime performed sufficent penance for their own sins 100 times over, plus they also suffered martyrdom. They could communicate this excess of penitential acts to other members of the body of Christ by applying them to the other person. Or it could be communicated for them by the Pope or Bishops in view of the power of the keys (St. Matthew 16.19, 18.18) to bind and loose all things.
Here is where what St. Cyprian is doing comes into view. He is permitting the Martyrs to promise a share of their suffering and future prayers to the lapsed to allow the lapsed to complete their penance. The only difference here from an indulgence as practiced today is that the Martyr is not yet dead and so is granting his merits in person, and the forgiveness of penance is not for a set period of time. If we truly believe in the communion of saints, the first difference should be irrelevant. The second is also irrelevant, because St. Cyprian could have set the letters of peace as having a set term if he had so chosen.
I think what is probably confusing you is the notion that some had that you could collect indulgences of 80 days or 3 years or 15 years and that these represented a fixed reduction of time in purgatory. That is certainly not the case in reality because it is creating an exact identity between canonical penance and the temporal satisfaction due to God. The penance represents the Church's estimation of what the sinner owes God and the body of Christ for the disorders he introduced by his sins. The temporal punishment is God's reckoning in the scales of justice.
Also an indulgence applied towards purgatory is done so in the mode of a suffrage or prayer, and not after the manner of an absolution from temporal punishment. The indulgence pays what is necessary for the soul to come towards bliss - we don't presume to judge the sincerity of the dead in finalizing their penance as we might on earth in reducing penance for a sincere confession.