OK, so it's not a normal bike. Still, normal bike fitting rules will apply. When I did spin classes, I set my bike up as close as I could to my real bike, following this procedure:
How can I fitting my bike
You'll have to ignore most of what doesn't deal with the saddle position. On your bike, I'd move the bars until that U-shape points away from you, tilted up just a little and just grab them near the center or along the bars so you can rest your forearms on them. See if that works. If they go up and down, try putting them a few inches above the saddle. You said you can change the saddle. Numbness and pain in the butt is usually the result of a saddle which is too soft! You want a saddle which is so firm, you can just barely push your thumb into the padding. It needs to be wide enough to support your sit bones, yet not so wide that it rubs your inner thighs when pedaling.