What sort of guarantees were provided with the rebuild? A good engine builder should warrant their work, provide security for any part failures assuming you have followed their prescribed procedure for running in the car after the rebuild. When the engines we rebuilt into the MX5 race cars for example, there was a very specific process to bed them in. Several rev limits, oil changes and a few other running things but once followed all failures of components were covered with a complete rebuild guarantee. I don't know how much money you have put into this car, but I would want to see the engine builder stand by their work if they are a quality outfit and put it right at their expenses if you can prove no miss use. I do not categorise a track day as miss use either, assuming again you followed any prescribed procedures. I would be looking for less mitigation of blame from the builder and more balls standing by their work. Seems you have had a bad rebuild to me that has gone wrong. If the engine is difficult to rebuild that is fine, but the engine builder either does a job with confidence or doesn't do the job. If they do it I expect it to be within the confines of a high quality warranty for their work.
I know nothing about the garage, the parts, the costs of the regime around the rebuild, but it does seems to me, after a scan read, that an engine has been badly built, fiddled with badly and has gone pop. If it were me, I would seek recourse in whatever form was needed.