I've taken some time to try out some of the suggestions you made. I've narrowed it down to trying JSWolf's ChareInk font. I certainly like this font, the NoWeight version is very close to the original Charis SIL font on my system. It's really very hard to notice any difference. The added weight version is, however, a lot too coarse to my taste. It gives the characters a bold appearance. The ChareInk font seems to completely eliminate the split lines problem when used in epub books that were originally plagued with this unfortunate condition when using the Charis SIL font. I think that generally speaking the Charis SIL font is slightly better in some characters, but the difference is really hard to notice and to complain would be like nitpicking. So from now on I'm going to use the ChareInk NoWeight font whenever the split lines problem occurs in an ebook.
I want to express a heartfelt
to everyone who participated in this thread. I felt real commitment from all participants trying to solve this problem, which was a really good experience. In my opinion this problem has been completely solved, leaving me to say:
