The immediate solution to your problem is to add this line:
\override TextScript #'self-alignment-X = #CENTER
If you'd like the text to be vertically in line as well:
\override TextScript #'padding = #3
You don't have to worry about these issues if you attach the analysis as lyrics. Then each markup is centered by default and all text appears in a single line. (You'll also find that the inversion figures are better spaced.) I've attached a file (including the \rN function) which adapts your example.