Regarding your archive, I assume you have your images in one or more folders and they are geotagged.
Using the Python scripts you can split your sequences. You can specify both cut time and distance, so if you have spend 30 min taking each image in a sequence of 10, you will get a sequence of 10. But the splitter will sort the images by date and time before splitting, so images taken in the same area on e.g. different days will not be put into the same sequence.
As you have written, Mapillary is sequence oriented, so stand alone images will often be put into a lot of single image sequences. That is also ok. Mapillarys algorithms will see that pictures are close together and add navigation between them, if they are close enough, no matter when they are taken.
I have never used the Panoramio importer, so I don't know anything about it.
Regarding only making script, it usually more than doubles the development time just to add a gui. Mapillary have chosen to make a web GUI for people who needs that, but provides scripts for power users. That is a choice. Making a nice package of the scripts also takes a lot of time. Again that is a choice.
Regarding choice of programming language, there are various issues with all of them regarding distribution. Spending a lot of time on packaging, like some big commercial programs do, would fix that.