Not sure if you're still having problems with selecting and deleting grass clumps. What I've done in the past is to just load up the grass plugin and click and drag over areas where I don't want grass to be and then pressing the delete key. Doesn't matter if trees, buildings or anything else get deleted as...
When I've tidied up the grass .ESP I'll process it in tes3cmd using, for example:
tes3cmd delete --instance-no-match Vurt_GRS_ "Vurt's Groundcover - Solstheim [Lush version].esp"
Basically, anything in the plugin that doesn't match the name of your grass objects (which in that plugin all begin with "Vurt_GRS_") is removed from the plugin.
So, all evidence of my messing editing is removed and only grass is affected by it.
One thing though: I did need to add dependencies to the Bethesda ESMs before tes3cmd would process it (and the Tamriel Rebuilt ESMs for mods that add grass to TR; not sure if you've made one of those...)