I guess Neptis sappho is not a very rare species, may be even more common than Neptis hylas in some areas of our region..but perhaps we often misjudge them...However now I am confident I will be able to differentiate them now with your described keys...
The wingshape is distorted ie held slightly up so all the markings are shortened.
The features that indicate this is N. hylas kamarupa are.
1.The FW apex in ssp kamarupa typically lack the topmost submarginal spot ;this is almost always present in sappho.
2. The upper two of the FW postdiscal spots spots 2 & 3,are very close ; they are well-spaced out in sappho.
3. The triangular cell-end spot is short although this is due partly to distortion ; with a sharper point in sappho.
The ssp varmona of N. hylas in central & south India is different.
1.The FW apex is fully spotted (submarginally)
2. The topmost of th FW postdiscal spot (spot 3) is often short. ( Note the postdiscal spots are in spaces 1a, 1b, 2, & 3.) http://www.ifoundbutterflies.org/#!/sp/480/Neptis-hylas
The last one (no.6) possibly be a variation of N. hylas, though the upper two poardiscal spots looks well separated...as the spot at 2 is very elongated, so i think it is a hylas...need your expert views on that sir..
5 & 6 are both N. hylas.
There is a fair bit of variation. Also two ssp. armona & kamarupa probably overlaps in your area.
The ID features for N. sappho astola whose features are more stable.
1. Markings are typically narrower & small.
2. FW apex ; the submarginal spots are almost always full ie right to the top.
3. FW postsiscal spots are mostly rounded & well separated ; none are wide ie elongate sideway.
4. The HW series of spots (upperside) are low & flat. http://yutaka.it-n.jp/lim1/720140010.html
Things are very much clear now for me...though i know there will always be some individuals that will create confusion first, but after gaining so much of knowledge from you about these two similar looking species, I am now much more confident about their identification...