The file locking problem is solved.
Still there is a timing problem now. If it takes too long for Maxima to produce the image, then the image region does not find it or uses an existing old version.
For complex images, this renders the temporary image storage in the Maxima plugin dir useless, because the name is changed everytime the image is written.
In the attached example, you can try that.
If the name parameter is set to "mpc" (anything without name extension produces svg output) or "mpc.png", then a png- or svg file is created (as can be seen in the file manager). However, it is not displayed unless the sheet is re-calculated. If recalculation is triggered by selecting materials in the (now correctly storing settings) checkboxlist region, then the old image is displayed.
If the name argument is completely removed, then temporary names are used and nothing is displayed at all.
I see two ways to avoid this problem:
a) Maxima has to wait until the image file is written and only then return control to SMath (sounds like a slow down)
b) The image region has to check on a regular basis if the file has changed. I don't know if that is possible.