Hi, so I assume you mean the threading that's happening inside of BLAS dgemm? That's the only place that ITensor uses multi threading right now. If you want to force BLAS to be single threaded, it's sort of implementation dependent. Usually the way to do it is to set an environment variable such as export OMPNUMTHREADS=1 which works for Apple vecLib / Accelerate and for MKL. (Also MKL recognizes MKLNUMTHREADS.)
Regarding 1-indexing, no it's just how our interface works, so it can't be turned off. We are actually considering making a Julia version of ITensor at some point where everything, even low-level things, are 1-indexed and then the indexing will be more consistent throughout the high and low level parts of the library.