One other thought. Do you have a hydraulic manifold under the pump? It will have a couple of knobs that close the hydraulic circuits to individual slides. That way you can keep one slide for moving while operating the others. I doubt this as the slides go out. If the valves were in play, the slides shouldn't move at all. Check out this Thread;
Battery
There being no resistance when trying to retract the slides is not good. I suspect a broken hose in the return circuit. Did you notice the hydraulic fluid level going down as you tried to retract? Is there a pool of hydraulic fluid or major sagging of the coroplast after trying to retract? Not knowing how empty the reservoir was and how much air was introduced in the line, it could take numerous cycles of in and out to clear the air out. But as you can't get it to go in, that is a challenge.
You say the dealer sold this to you knowing the slide switches didn't work. They provided a "magic" wire to extend the slides but no information on every retracting them. I am reluctant to suggest further tricks as there are some expensive parts there (pump motor = ~$$900 / Reverse Polarity Switch ~$100 / DPDT switch ~$50 / and more). Have you asked them for help?
As for the rams, they are going to be hard/impossible to move while while hydraulic pressure exists. Those "nuts on the exposed ram are adjustments for how far the slide comes in and goes out. Removing the nuts means*YOU* will have to adjust those distances yourself later. Yet something else to learn under pressure. And even if you disconnect the rams, they wont move while pressure exists. They will stick out as they do now and be a problem even if the slide avoids them.
If you are determined to do this yourself, not knowing what you don't know, disconnect the hydraulic lines connected to the rams. This will allow the hydraulic fluid to escape, thereby reducing the resistance and creating a mess. But theoretically, it will allow a group of folks to push the slides in/out. You can buy caps and plugs for the hoses and rams to try and minimize the loss of fluid. Look here:
Disconnecting a Hydraulic Hose ???