The judges only look at the tech panel's decision? Can they question back if they see some unreasonable calls?
The judges never see the level calls during the competition. For spins and steps, the judges are only responsible for evaluating how well the elements were done and how they fit into the program as a whole (e.g., choreography) -- the difficulty is not their responsibility under IJS. There are so many details for tech panels to learn and remember that no judge is also going to keep track of the features at the same time that they're judging the quality, unless they are also technical controllers or technical specialists and have done those tasks so often that it has become second nature to them.
The judges do see the <, <<, e, and ! calls at the end of each program, and they may also see the new s and ss calls. For the jump calls, they are required to reduce the GOE from what they would have given without those errors, and for << or e calls they are required to give negative GOE (no higher than -1).
If the judge saw an error that wasn't called, that judge is welcome to reduce the GOE more severely, based on what they actually saw.
If they did not see an error that was called, they can be more generous with their GOE reductions. E.g., if the judge thought "That was a great lutz! I would give it at least +2, maybe even +3" and then the tech panel calls it as ! and <, the judge can still give it +1.
If the tech panel called e and <<, the base value would only be 50% of the total, but the judge could remain generous, awarding all the positive bullet points before reducing down to -1, rather than -3 as would be expected for both those errors and
However, there isn't any mechanism for one judge or even all the judges to say "That jump looked fully rotated, or close to it, and the takeoff edge looked correct. We have replay too, we reviewed it, and we didn't see the errors at all, or didn't see them as being severe. The calls really should have been ! and <, if anything, not e and <<, and therefore the skater should have gotten full base value. You were wrong, tech panel -- go make the change officially and give that skater back the points she deserved."
The best the judges can do for a skater in that situation is to give the highest appropriate GOE for the element, and not to penalize at all in PCS. If a jump with an e call is nonetheless beautiful, judges can also take that beauty into account under the Performance/Execution component.
So a level 4 spin with 50% deductions is worth as little/less than a level B spin with no deductions?
hwell: 
