Well, we finally solved our issue. It took longer than i would have hoped to get an opening on the machine but this weekend we got it all straightened out.
We started by loosening the base bolts to the table (which we had tightened the S*** out of initially - not good). we trammed the table and the measurements changed so it looked promising. We then removed all of the shims we had put under the base. Then we lifter the front of the mill and supported it with a .75" aluminum block in the center. This holds the table in a three point support and can't flex it at all as many on here have said to do. We then trammed in this position (still with all base bolts loose). It actually looked much better, but when we would get to a certain point the measurement would jump. It turns out the Y gib was now loose (no longer pinched from the base flexing), so i checked the X gib (did not need adjustment) and re-adjusted with Y gib, all while still supporting the mill in three points.
Once all gibs were re-adjusted it was time to re-tram the table...and WOW what a difference it made. We saw 0.0005" across the entire 18" X travel, and no noticeable change in the Interapid indicator across the 9.5" Y travel. we were ecstatic to say the least!
Now it was time to set it back on all 4 feet. We removed the block that was supporting the front in the center and set the mill back on all 4 feet. We trammed the table and saw about 0.0015 across the 18" of X travel and about 0.0006 Across the 9.5" of Y travel. We wanted to do better. Our approach was to lift the front and insert a shim under ONE of the feet and then set the mill back down and re-tram. We guessed the foot right the first time but the shim thickness was too thick (0.030") and it twisted the table the other way. You can tell by the direction of the indicator when you tram the Y on the left edge of X travel vs. the right edge of X travel, we reduced the shim thickness to a 0.010 shim and it was much better, but we wanted perfection. So we changed to a 0.007" shim and that was the one! we have 0.0005" in the Y direction on the left edge, 0.00125 across the 18" in the X direction (both top and bottom of the table), and no real noticeable change in the Y direction in the center and right edges. We decided NOT to tighten the base bolts, and just left them finger tight.
The machine has been running since saturday and i just re-checked it last night and it still trams as good as it did when we adjusted. I will check it again in a few months to make sure.
In addition the center T slot came back to life also, the FRONT edge (the ground one) measures with no noticeable error until the far right hand side two inches, and then it changes by 0.0005", so it looks like we can just set vices and fixtures on for standard tolerance work. and henceforth i will return to calling the center slot a "Precision center slot"
I was so used to working with the crooked table that everything we have made since has seemed like cake, many of them coming out within +/-0.001 in Z depth across the entire thing (usually 10.25"). So at the very least i may have gained some skills trying to compensate for the crooked table for the last several months lol
Sorry if this was long winded but i wanted to document what we found and hopefully our struggles and what we learned will be of use to someone in a similar situation in the future.
Thank you again everyone for all of the helpful input and suggestions while we were trying to figure this out.