I can sort of fix the non-importing forum by moving one of the topics to another forum. Then, everything imports fine and I just need to move the topic back to its proper place in SP.
And I guess I could fix my other problem by doing a find & replace in MySQL on the character used as an apostrophe by the Mexican poster, so for me, it looks like I have a solution.
The offending character is 0xB4 while it should be 0x27, AFAICS.
I’ll leave my DB as it is for the moment though, in case Andy still wants to play with it.
There is still the 2 issues with the current beta SP5 importer.
-Directory needs to be renamed to sp-import before it will run
-Doesn’t import any posts.