So it seems to be be working properly lands you back on the Parent record of the one you just saved, which in your case it actually the child record of the main Parent project.
There are two choices
1. Just have a link on the feeder record and the user will then navigate back to the parent. But i guess that you don't link that option.
The other options that should work is to make a formula field like this to Add Regulator.
var text ADDRegulator = ( and then paste in here the same formula that you have now but without the last last which is like & & "&z=" & Rurl() (be sure to end with a semicolon ;
var text DisplayProject = urlroot() & "db/" & [_DBID_PROJECTS] & "?a=dr&rid=" & [Related Project];
$ADDRegular
& "&NextURL=" & URLEncode($DisplayProject)
