In general, we cannot guarantee this kind of compatibility (which is called "forward compatibility"). In practice, SimBiology models have been largely compatible across releases. Hovwever, R2019b introduced a major change that means you cannot load models in earlier releases.
So, regarding question 1: There is no way to directly load a R2019b SimBiology model in an earlier release. You must save/export that model in some special way if you want to use it in an earlier release.
Regarding question 2: The simplest option for sharing a R2019b model in an earlier release is to export the model to SBML. However, not all features of SimBiology models are compatible with SBML. If you want to preserve more of the model across MATLAB versions, the best way is to transfer the model as "code". By that I mean you can write a script that constructs the model using the same commands you use to construct it in R2019b. If you need to do this, consider contacting MathWorks Technical Support. We have some automated tools that could help with the process.