Hi and welcome to Arturia forums.
Are your ARP 2600 in MONO mode? I assume you are.
And i understand, that you don't wan't to use Pitch bender and that include Tuning parameters i assume.
The automation parameter "Portamento" controls the on/ off state of the Portamento.
The automation parameter "Portamento Time" controls the time/ speed/ rate of the Portamento. It does'nt change the direction of the Portamento. I think you might misunderstand how the automation parameter "Portamento Time" works. It look like you think of it as a position and not a time/ Speed/ Rate.
It's notes that activate portamento and the direction of the portamento.
If i understand you correct about what you wan't to do, then i would:
1. First be sure B1 was the last note played, then activate Portamento. This to ensure B1 is the starting point of the Portamento.
2. Then when E2 is engaged the Portamento start. The Portamento Time ishould be set to reach E2 where you wan't the Portamento to change direction. I understand you wan't E2 to be reached.
3. At that point you should play D2. Then the Portamento will start glide to D2 from E2. If you wan't a change in the glide time at this point, then you can set a new Portamento Time for this glide, so the note D2 will be reached at the time you wan't it to be reached.
You have the E2 playing to long time. Set the D2 to play when E2 is reached, - the point where you want to start the glide to D2, and shorten the E2 note to that same point.
Hope this helps. Feel free to ask again or for more, if you need it.