I think the whole "mold release" thing is exaggerated, but it is very hard to get to the bottom of this- I did a fair amount of research on older vinyl compounds and found several papers published by the AES and some information in patents that described what goes into PVC for records. The formulations are often proprietary, so it's hard to pin down exactly what is in a given product, and presumably, a label or plant could order something that is different, from a different supplier, or with a different make up, than another record plant. (I don't know if you remember the whole thing about "Clarity" vinyl toward the end of Classic Records' production, but Mike Hobson claimed that eliminating the carbon black--used for antistatic purposes--improved the sound because it had electro-magnetic properties that interfered with the performance of the cartridge).
As to cleaning, it seems to be a subject that everyone has a different view of- if your Speakers Corner copies play cleanly out of the sleeve, enjoy them. You might also investigate some of the original pressings that they remastered for their catalog. Although some are expensive, others aren't and if you are interested in what the differences in different masterings, tape sources and the like contribute to the end result, sonically, it is actually a fun process to compare pressings.

