About user map: Just rightclick in each generated project map and choose "Add to map" -> All Components -> "New User Map..." (change the name if you want to.
After you have done this with each project (15?) you have one single large user map. Right click on/in that user map and choose "Write to file"
You can then create one single at5-file.
If you create atlas version 10 and only use "safe colours" you can try the map in an emulator (
www.lowrance.com/en-US/Support/Product-Emulators/) I recommend GlobalMapĀ® 9200C Emulator due to the screen size. There are no (official) HDS-emulators. The emulators cannot do raster backgrounds.
User maps can be very large without issues.
Anoother way to combine all parts in one at5 file is to export each project to shapefiles and then in IMC manually load all these into one "IMC-project" and generate it in one go.
I use a combination of these actions. First combine very large lakes into one usermap. Then combine all lakes/"User maps" into one at5. Not exactly "all", but rather those in the same area.