As default, the model discretization is set to be rectangular.
It is possible to change to the trapezoidal model discretization by clicking 'Inversion - Model discretization - Use extended model' option and select 'No'. After that run the inversion. You can change the Res2dinv.ivp file using a text editor if you want to change the default settings, just do a search for the word 'extended' to find the setting.
The reason for the rectangular configuration of the inversion result opposed to the trapezoidal shape, is that there is no reason for the inverted profile to be trapezoidal. A pseudo section is displayed as a trapezoidal shape because it is a crude approximation where each electrode configuration is assigned a focus point at which its apparent resistivity is plotted, as the deeper focus points requires greater electrode separation the trapezoidal shape is a result of this way of displaying data. An inversion, however, models the resistivity distribution under the profile in its entire length (actually also some distance beyond the profile) to find the distribution of resistivities that is most likely to result in the measured data. Therefore the inversion result can easily contain information outside of the area covered by the pseudo section, and there is no reason to not to display this information.
To see the sensitivity distribution or to see how well resolved different parts of the profile are you can enter the display sections mode (under “display->show inversion result”) and choose between the different sensitivity displays under “Display section->sensitivity display” where you can choose to plot the uncertainty of the different parts of the profile. You can read more about how these are calculated and can be used in the manual found in the installation folder or here: https://www.aarhusgeosoftware.dk/res2dinv-res3dinv