Well, I've been playing for a while with the new RMSDK renderer and the issues I knew about it and the results are somehow mixed:
Strange image rendering of certain png images -- Fixed
This issue (which also happened in my old PRS-650) seems fixed.
Lack of "font-variant: small-caps" support -- Not fixed
(As sadly expected

)
In this case ACCESS supports SmallCaps...
"white-space: nowrap" broken -- Not fixed
As I explained in the
Kobo bug thread, this trick I
always use around Spanish dialogue dashes (usage detailed in
this Calibre thread) has stopped working in RMSDK. And it works in either my old Sony PRS-650 or in ACCESS kepub renderer!!!
A pity because it is a stopper error for me.
(OTOH ACCESS has a horrible space added around dashes and other punctuation marks... Kindle maybe the solution for me?)
Widows and orphans -- Buggy
Yes, the long paragraph bug has been vanquished (at last), but the widows and orphans settings are somehow buggy.
Look at the three pictures I upload (three consecutive page-turns). I can assure you that the book has no CSS settings about them, so the default value 2 should be applied.
Within the first and the second picture you can see how a fully filled page looks (the first one), and how 2 seems to be working in the first paragraph of the second picture.
But then look at the incredible big space at the bottom of that second picture, while on the third picture that paragraph has THREE trailing lines!!!
Why didn't RMSDK put the first one of those last three lines at the end of the second page?
(A pity ACCESS doesn't support widows and orphans in any way... Again, is Kindle the solution for me?)
So considering all things together I will continue in ACCESS with sideloaded kepubs through KoboTouch Extended...