Btw, on my work we haven't admin rights too. What I did in this case:
0. Save sqlnet.ora, tnsnames.ora
1. I asked corp admins to completely wipe huge (>1.2Gb) oracle client from disk and clean registry keys named "oracle".
2. Then download from oracle.com & unpack "instant client basic lite"+sqlpuls+sqlldr (if needed), to the path where you have full access. It consumed only a 125Mb of disk space.
* notice that it doesn't contain tnsping!
3. Create "NLS_LANG" environment variable corresponding to our database.
4. Move saved sqlnet.ora, tnsnames.ora to .....\instantclient_18_5\network\admin\
5. Choose correct path to oci library (e.g. d:\oracle\instantclient_18_5\oci.dll) in PLSQLdev preferences\connection settings.
That's all!
I use that config & scenario also on home computer under Win7 connecting to Oracle Cloud Database.
And it is strange, why you don't have access to user's environment vars. In my work's Win10, I opens the Windows' "Start" menu and type "environment", then choose "User environment" in search results, and I may change them on my own.