I'm trying to figure out some of the responses, but I got a headache so I'll give it a go...
On your chainline, if your bike is setup correctly now, in theory the chain will the perpendicular to the hub when you are in the middle chainring and on the 5th cog (middle of the cassette). If Stronglight recommends a 115mm it is based on trying to the get middle chainring to be lined up to the middle of the cassette. There may be quirks to each bike so it may not be a perfect alignment.
Because you are only going to use the inside and middle chainrings, in theory, the middle alignment is technically centered between the inside and middle chainrings. The reason why some people are suggesting a 118mm is to push the crank further out and try to get the chainline centered between the inside and middle ring. I don't know why Treefort is suggesting 113mm. If it is based being familiar with your setup, then it is sound advice. If it is because it is close enough, then I would probably go 115mm. Keep in mind, all the best math sometimes doesn't play out on practice.
As far as shifting, you don't do anything but set the limit screw to block out the outside (large) chainring. There is no tricking 1 becomes 2, or 2 becomes 1.. 1 is 1 and 2 is 2 and there is no 3. Apart from adjusting the front derailleur for the new setup, there is little you need to do.
John