I have done this several times and it is not very hard. First off There is two ways I have done this. I remove the pin from the piston and only one ring from the pin. I put the piston back in using a ring compressor then push it to the bottom of the cyinder just enough for the pin to go back in then I put the ring in holding the pin. The I ease the cylinder back down on the piston. But you have to be very carefull doing this.
The second way is if the cylinder has a tapper on the bottom which most do have. You can take your fingers and compress the rings as you slide the cylinder down. I have done this several times when the piston had to came back down to far and exposed the oil rings before the pin would go in.
I do this when there is no problem inside the engine and just reringing the piston. If there is any damage to the piston or the cylinder a complete rebuild would be in order. The rings are harder than the cylinder wall so if you are careful they will not be damaged. I have never heard of the piston has to be inserted from the top side.
I have rebuild several twin engine mowers and never had a problem. Any engine with new rings if not broke in correctly will start using oil. My customers have been running engines I rebuilt like for years with no problems. Just because it is a redneck way does not mean it is a bad way. My father was a master mechanic and showed me several short cuts that work but is not in manuals.
I appreciate what you are saying, however you have to remember the target audience .
I also appreciate that in a full time workshop the situation is vastly different to in the shed of a very much amateur .
In my workshop I can ( usually ) do a full & proper diagnosis so end up knowing exactly what needs to be done and if I am busy limit the job to just that .
However I do like to do most jobs the long way so I can do things like examine to bore & pistons fully which is difficult to do with the rod still in the engine .
So while you pappys short cuts are a valid way of working , they are for discussion amongst professional or fully competant engine rebuilders and not on an open public forum where the bulk of the people are total novices.
As you know the rods on these engines are quite frail and easily twisted or bent by people whacking the pin with hammer .
I have a box full of pin clamps ( somewhere ) to push pins out without damage but I doubt that any standard home owner would have them.
Also there is a big difference between working on a vertical twin & a car engine.
By the time the engine is pulled from the mower , sump is off , head is off & rod cap is loosened /off there is not much to be gained by not pushing the piston all the way out .
Again if there is 20 more to be done then yes the 5 to 15 minutes of time saved would be significant but on a single one off job not really .