No, you can't run a "one-way" in reverse.
The spring may not sound like it makes a difference, but it does.
Make sure you have ALL the parts required.
Make sure you've relieved mat. around the center of the diff/ring gear
(as per the kit instructions).
Tight is tight. If the diff locks down, the screw and nut are fine.
Forward lock (assuming you've got all parts in there--assembled correctly)
has everything to do with the right grease (it must be DIFF lube) and good spring. Silicone grease, Black grease, etc. will not work on the main balls.
AE makes STEALTH diff grease (clear) and SILICONE grease (also clear). Don't get these mixed up. One tube's writing is in black (stealth) and one is in red (silicone).
Just methodically work your way through the instructions--following everything EXACTLY. I bet you find the answer.
I ran for AE for several years--and ran the car for a few years prior, as well. Those diffs are very dependable.
Team Schumacher - Team Powers - EDS Specialized RC Tools - RCSpeedshop.net