First thing to do is reconnect the climate control vacuum line. That could easily be the source of the transmission problem. If you had a Mity Vac, you'd T into the line going to the transmission modulator from the VCV.
You can purchase a reasonably priced Mity Vac from Harbor Freight or Sears. It's essential for diagnosing vacuum problems on these cars.

Is the 3/2 Vavle necessary for shifting? What if I was to bypass it?

Don't mess with the 3/2 valve until you get the climate control line reconnected. Your problem is most likely caused by a vacuum leak somewhere.

The climate control tube wasn't originally connected though? Do you think changing the 3/2 valve somehow changed something that means it needs to be connected?

Huh? Need a picture. Shouldn't be loose green vacuum lines hanging around. Did someone have the line which the green line connected to plugged with a golf tee or something and now the plug has fallen out?

To OP:
The forum is only good for piecemeal procedures on this project. You need trans diagnosis instructions for your specific trans. Begin by finding out what trans it is. Most have the number on the passenger side right above the pan.

ATSG has good manuals. Professor Google turns up lots of relevant info. There is also information on youtube. One link leads to another until you understand. It isn't difficult. It also isn't easy to explain thoroughly in a post. Do not go haphazardly or you will mess up more than you fix and wander aimlessly through the woods.

Edit: Transtec.com has a pdf that will be helpful. Use the link to search Mercedes transmissions. You need to know what trans you have ~ (722.xxx ?)

The problem began suddenly after someone messed with the vacuum lines. The chances of it being caused by a problem in the vacuum system caused by this recent work are extremely high.

Mity vacs are always on ebay too. If you have time to be patient, they can be very cheeep there too.