I am not disagreeing with the main thread of your argument but I should make the point, that in fresh plumage at least, Japanese Leaf should be diagnosable in the hand to an experienced observer. I was recently in conversation with Phil Round on the very subject had here is what he had to say:
'JLW is larger than KLW as you say, with a markedly thicker bill. There is overlap in wing length, at least, between the largest (male) Kamchatka and the smallest (female) Japanese, just as there is, say, between large male Arctic and female Kamchatka. The bill of Japanese is markedly stouter Calls are diagnostic.
I have now had four Japanese LW in the hand and besides being obviously brighter, more or less evenly yellow on the underparts, they are obvious on size right away. Even a (relatively short-winged -70.5 mm-by Japanese LW standards) female was obviously a large warbler right away. But it can be surprisingly difficult to see the extent of yellow in the field, with reflected green light off foliage (from observations on a newly released bird that was subsequently watched, perched, while preening after release).'
It's worth restating his word of caution re field ID; the amount of yellow saturation, which includes the ventral region, is a key feature but it can be tricky to see in the field, equally these yellow tones are often 'lost' in digital images which presents its own problem with regards image-based identification.
In short, like you, I would not advocate making a firm ID from images in the absence of a sound recording.
Some useful reading
https://www.researchgate.net/publication/280383942_A_record_of_Japanese_Leaf_Warbler_Phylloscopus_xanthodryas_in_Thailand and an autumn individual here for comparison; the heavy bill is obvious but the yellow tones are rather more subdued though there is no way of knowing whether or not this is a true likeness
http://orientalbirdimages.org/search.php?Bird_ID=1808&Location=
Grahame
