Re: your first question, the credit card processing functionality is the same regardless if using PCCharge or SPS. However, if Sage 100 shows the original invoice as being paid via credit card and you wish to issue the credit against that original credit card, you should simply be able to enter the S/O CM and select the original invoice in the Apply To field - and the original cc info should present itself. Otherwise it's likely still a 2-step process.
Not sure I follow your 2nd question.