Not knowing and seeing or hearing both side of the argument there can not make a proper assessment of what happened . There are just to many Questions. The only resolve is to attempt to try again and work with all parties involved in the dispute and make the best attempts to keep it civil. Since I don’t know the actual chain of events in making the purchase or the rebuilding parties then it is only up to you and those parties to come to a civil and proper compromise to settle this .
Questions like why gear oil and why AFT 4 … My opinion is that both are lubricants and either or should and would have lubricated the gears . I’ll say I’m not familiar with the Manual transmissions internal working parts or a requirement using ATF-4 and I can only assume since gear oils have been used for this purpose for hundreds of years with no problems . The catastrophic failure appears to have been caused by lack of lubricant no matter what lubricant was used IMO !
A leak of some sort was obvious as you stated , Why did it loose oil , a bad seal ? A cracked case ? Before or after the install ? It does make one think that when installed and fluid added ,,, seals , cracked case etc would have shown some leakage pretty much immediately and as it lost more and more fluid , lack of lubricant and hauling ass at 120 mph for a 400 mile run was just the recipe to a catastrophic failure ! You would also think that you would have smelled some sort of oil burning as it burned off the exhaust. I’ll give you the benefit that at 120 mph and night time driving you probably didn’t notice any smoke !
Like I’ve stated , the only options are to make and continue to open a dialogue with Dan , the rebuilder to comprise! Without running the gamut you may or may not eat an expensive cluster phuc . Nothing will ever get solved without giving it your best and persistence. I just don’t see any benefits posting up on forums and slamming anyone whether you’re in the right or in the wrong . Others can not make a one sided to take one side or another !