It would be far more useful if you could provide explanations of what the problem is, why it is a problem

Each problem came with code to demonstrates it.

and ideally some hints on how to fix it.

Magic: Use SvGETMAGIC(sv) before reading from an SV. Use SvSETMAGIC(sv) after writing to an SV.

Unicode bug: Just like numbers can be stored in a scalar in a number of ways, so can strings. Accessing a string's buffer without determining how data is stored in it is a bug. (This pretty much rules out using `char *` in XS function prototypes.) If you expect bytes, SvPVbyte will get them. If you expect text, SvPVutf8 will get it encoded using utf8.