Merged and released in 1.5.2

dalvizu sorry for the delay I was in holiday.
About issue #1 yes you understood correctly, I see, so I have to run the job from the build pipeline view or it will not work (acceptable even though this way there's no way to run parameterised jobs).
About issue #2 this is a show stopper for me because, since I have to run from the view, the job fails with the error of missing BUILD_USER_ID (https://wiki.jenkins-ci.org/display/JENKINS/Build+User+Vars+Plugin doesn't seem to work when running the job from the view)
I'm running Jenkins LTS 1.642.3 and the just released version 1.5.2 of the Build Pipeline Plugin.
I see you tested Jenkins 1.646 and 1.654 though saying that Build User Vars Plugin works there, if that is the case I'll just have to wait until they update the LTS.
Many thanks for your support.

The testing I did was:

Create upstream job
Create downstream job

The only time 'BUILD_USER_ID' appears in the downstream job is if i check 'Set Jenkins User Variables' in the downstream job. In my testing it is never passed down from the upstream job. Do you know of a version where this isn't the case?

If you can reproduce please include the job config.xml and the versions you used so I can take a look.

Thanks dalvizu for your help, I finally managed to have the Build User Vars Plugin working properly also when manually triggering the build from the view.
I started testing on a virtual machine with Ubuntu 14.04 LTS with Jenkins 1.642.4 and it worked immediately.
After upgrading the production environment to Jenkins 1.642.4 anyway it didn't work immediately, I made many tests trying to understand what was wrong, it seems to me that it started to work after I deleted every single build from the downstream manually triggered job and then from the view I triggered the build that went green with the BUILD_USER_ID variable properly created.
Thanks again for the support.