I have tried your installer, and it seems to me that only it installs the version GDI and the pak 64 from Internet, in addition, had a nightly put with the name simutrans.exe instead of simwin-sdl, and it has rewritten above. You will see that in STIUP, it will never install a version previous to the one that is had. As for doing an installation online, it is what I was developing as Simutrans updater.
An image, two:


The project was going to be: an installer offline (there are people who they do not have an Internet conection) and an updater, since what you see in the images, it cannot install online, only it rewrites the files (there must exist the files of the previous version), since it gives a critical error. At the moment, which you see is a test that I did with 20 % of the files, since I have to mess about on many lines.
I am sorry not to be able to help him with NSIS, alone I know of inno. But I believe that there is a convertor inno-NSIS that can help to understand both codes from other perspectives.
Summarizing, to publish it, it would have to be without the content offline (Simutrans files), only it is possible to arrange an download. But it was too complicated because I do not know completely the functioning of inno setup.
This is the backup of my installer, with the distribution of Simutrans's files for the installer, the code and the programs necessary to compile it.
STIUP codeIt's protected with password.
The password is: y5EJq2Cr