Currently, the only possibility to configure the engine used by LivePreview is a per file selection.
Additionally, this possibility seems to be buggy as some year old bug reports indicate.
First, in projects with a master file, not the master files value but the one of the currently opened document is applied. (Bug 379663)
Second, the per file setting seems not to be persisted. One has to reconfigure the values after every start. (See e.g. Bugs 408890, 307437, 435804 )

Most of us users typically use one TeX engine predominantly or only use one at all.
If it is not PDFLaTex (which is quite often the case, especially if you need unicode support) one has to reconfigure the setting after every start. This is quite annoying.

A good workaround would be if one could at least configure the default engine which is active at start.
Because usually most projects use the same engine. So it's easy to just configure this once and be done with it.
The above said is also true regarding the bibliography engine. If one uses biber instead of bibtex, one probably does that for most uses cases.

So an option to configure the LivePreview and bibliography toolset globally would be a great new feature.