Do we know exactly which combination of Cairo and Inkscape makes SVG
Fonts dialog working?

I dont know

If we keep SVG Fonts for 0.47,

yes, we will :-)

could we please have a little more
verbose UI?

where would you put these instructions?

At least mentioning which way exactly of entering Unicode
value of a glyph,

I have changed the name of that column which was previously called "Unicode". Now it is called "Matching String"

and a spinbox for precise setting of kerning.

OK, I will try to work on that now.