[noggin]

So - does Kodi follow the following logic ?

* 25p native content - output at 25p if 25p supported, otherwise output at 50p with 2:2 frame repetition if supported, otherwise output at 50i with 2:2 pulldown and interlace if supported (and ideally a vertical filter)?

* 50i native content - output at 50p deinterlaced from 50i if 50p supported, otherwise output at 50i interlaced from 50p deinterlaced from 50i if 50i output supported (i.e.deinterlace and reinterlace rather than passthrough interlace)?

* 50p native content - output at 50p if 50p supported, otherwise output at 50i with an interlace (and ideally a vertical filter).

* 25psf content - treated as 50i. (i.e. 25p acquired content carried as 50i with no motion between fields - aka 25p segmented field)

Most 50i broadcasters use 25psf for HD content - but some final-link encoders (like those used in Freeview HD in the UK) will switch the encoder between 25p and 50i paths on a GOP-by-GOP basis based on whether the encoder thinks the source is 50i native or 25p native (carried as psf)

I have only ever come across 25p format support in the context of 2160p - not sure I've seen it in 1080p on TVs.

Nope. Use the highest refreshrate in the given resolution for which fps % refreshrate == 0.0 - if possible.
If you got a hdmi 1.4 only with max 3840x2160@30 - we play e.g. 3840x2160@25p content at 3840x2160@25 if available and don't use the 1080p50. Check BaseRender.cpp - I tried to comment that nicely last time I hit that file.
And to add: we never switch to an interlaced mode.

So if Kodi is connected to a 720/50p and 1080/50i only display that doesn't support 1080/50p - what happens when you play 1080/50i content? (I potentially have this situation if I use Kodi at work where we only really handle 1080i and 720p video over HD-SDI)