Rolando,
I am not an expert on the printing process for this issue, but it is my understanding that the relief dies used to produce the ten different types for each denomination, in that each varied some from one to the other. However, when these dies were used to lay down the images they were always the same. What is referred to as retouches in these issues are those positions where the image wasn't removed and another put in, but actual retouching the image on the plate. If this were done, as long as that position image remained as part of the plate, there would be a constant flaw or retouched image.
This is where we have varieties beyond the actual relief image. So, it is possible to find say the SE spandrel redrawn on type 19 if the 25c (this is an actual retouch found on stone F, position 100). Anyway, there are a number of these that have been identified, by collectors that have found more than one of the same retouch.
We also have flaws that happened due to a transfer flaw. These type varieties also fall into the collected spectrum of constant flaws and retouches.
Now, regarding your second part of your question. My position on collecting these types and varieties is all positive. This has been done by the likes of Vannotti, de Smeth, Fayolle, Larranaga, Benson, Josling, Speaker, Heath, Stout and many more. This list reads like a Who's Who of Mexican Philately. Myself, if I were not so heavily involved in collecting the Dos Reales and selling classic stamps, I would have started a collection of the 1868s years ago. As a matter of fact, for years I just stock piled this issue, because they just weren't very good sellers; however, about 5 or 6 years ago, I discovered that indeed they had become better sellers with a new interest from many collectors. So, what use to be an issue collected by a very small circle, widened to a much larger group.
Surprisingly, this issue is still pretty easy to find and the average prices for many of these stamps is still low enough for most collectors to put together a fairly good collection. At this point I don't see this issue becoming anywhere as difficult as the first issue or the Eagles for a collector to assemble an outstanding group of these stamps. Also collecting varieties doesn't limit the collector as much as finding a rare district or rare cancel. Many of these varieties are on the very most common district and denominations.
It was very sad to lose our friend Doug Stout, but for the collectors of the 1868 issues, part of his collection is being put up for auction in the next Follansbee sale in October. Another part of it is to be auctioned off by S. Rumsey auctioneers. So, some pretty good material is ready to hit the market. We will see a lot more from his collection over the next few years.
So, yes, collecting these stamps and varieties is not only fun, it can be also be a collection you will be able to pursue for years.
PS. The image I put up was done by Manuel Iglesias from stamps in his collection. I only cropped and compressed the image to put it up on MexiMail. Indeed it is a great image.