Most drugs generally are salts such as HCl yes, but benzos are notoriously hard to make into salt and a lot even unrealistic though there are some exceptions - most are freebase instead. I don't believe diclazepam is an exception, so it would be a freebase just like I knew most benzo's I have had to be like alprazolam or diazepam powder.
They dissolve well in pure alcohol, though varyingly diluted alcohol solutions can be used. For example I could easily dilute my etizolam from 100% ethanol to 50% without any crashing back out again (I started at a concentration above 1 mg/ml so that diluting would end up at 1 mg/ml) and I believe 40% vodka also worked. However alcohol is volatile and water is too, kinda. I think PG is much less volatile and generally the resort of people wanting to dissolve the average fb benzo, though sometimes injecting is involved which makes it a different story altogether.
Read up a bit, apparently it can take a very long time for diclazepam to dissolve in PG even multiple days. I don't know if warming the solution helps (well I sort of do - it pretty much always does as a general rule of thumb of chemistry), if you do try don't go as far as boiling it.
I do not know the saturation point, choosing say 5 mg/ml may not be quite as hard as 1 mg/ml.