Jenkins workspace root getting overwritten or not accepted
That's the design of the p4 plugin. It puts the workspace where jenkins asks us to.
See property jenkins.model.Jenkins.workspacesDir here: https://wiki.jenkins.io/display/JENKINS/Features+controlled+by+system+properties
I don't think the default in that wiki is correct.
On all your master and slaves, you can try to change that to just D:\
That assumes your client view definitions (right hand side) will not overlap.
Otherwise:
A "form-in client" trigger script can alter the root. The script should only change jenkins relevant clients, so you'll need to pass something to the script in the trigger definition to signify that it is for a jenkins job. Examples could be a client naming convention and/or the clientip.
Your Perforce Admin, if that's not you, will have to assist.