For the first question, there is no method to prolong the trial period, but if you install a new version (a larger version number such as BestSync 2018), the trial period will prolong 7 days.
For the second problem, I think that you are using BestSync in a 32Bit OS, 32Bit application limits the usable memory to about 2 GB. Because BestSync is synchronizing with the folder that have too many files, it exhausted the usable memory, the problem happen.
If you use BestSync to synchronize large folders (contains more than 200,0000 files), you must use BestSync 64bit under 64Bit Windows.
Otherwise, you must setup many tasks to synchronize small folders (contain less files).