Satisfaction of Poincare-Hopf relationship is not needed to consider, since basin analysis ensures that all maxima and minima are found (as long as the grid spacing of the grid data is small enough).

To plot 3D isosurface map of ESP, you can use main function 5 to compute grid data of ESP, and select option -1 in post-process menu to visualize its isosurface. Also you can use other code to generate .cub file of ESP, and then load it into Multiwfn, then select main function 0 to visualize isosurface.

I have a blog article written in Chinese to show how to plot a map containing ESP extrema and ESP isosurfaces, it should be useful for you (you can use Google translator to understand the content if you cannot read Chinese):
"The method of plotting global minima of electrostatic potential + isosurface map to show the position of lone pair electrons"
//www.umsyar.com/493

I have two main questions:

a). How can I calculate the minumum and maximun critical points of the electrostatic potential?

I have used:

2. Topology analisys.
11 Delete results and reselect real space function, current:  8

and then I used option 2-6 to find the critical points.

¿Is this the correct procediment to find the ESP critical points?

I have used all the tips on the manual to find all critical points, but the  Poincare-Hopf relationship verification is still -11.

b) How can I plot the MEP 3D surfaces? I want to find qualitatively the critical points by varying the isovalues of the MEP surfaces.

Thank you in advance,