@
cybmole - you are wrong on both counts
For each format you can have many programs as you want:
As as I already said they can coexist - they can't use the same keyboard shortcuts, but what can.
There is one corner case where it won't work - it can't invoke shell extension DLLs, but then again I don't think the PI could either, so nothing lost or gained. Sometimes I need to rotate or mirror a cover image - I have a shell extension to do simple edits like that - but I don't need it often enough to bother KG, besides 'o' to Open book folder still works

Or I can do it IrfanView
Oh, yes - I think it integrates with Windows Default programs or something - so if a proggie is in your Windows Open With then it will be in your calibre Open With automagically, sadly it does not feed the cat or make the coffee - KG is working on that
BR