Only 1 extra text can be displayed below the Arabic verse.
That text can be a translation or a tafsir. The application doesn't even care or know. You can cycle between the available extra texts somehow using a way I don't know yet

Let's say you downloaded 10 translations. You enabled only 3 of them. You can cycle between all of them.

Let's say text a, b and c are enabled.
If you cycle and choose a then a becomes the default one. Next time you close and open the app a will be there by default. and so on.

I don't really understand what you want to say regarding the previous and next problem.

If you don't want to make icons and/or the configuration file for the theme. How can you create an additional theme then ? The ini file is needed for defining colors at least.