Reconciliation accepts negative Ending Balance Entry (for CC) but changes to Positive

That's the way it works. In Quicken, credit card accounts are like any other bank account which means when you spend money it's a negative transaction. That means your credit card account normally runs a negative balance indicating you owe the credit card company money. The balance on a credit card statement OTOH is from the bank's point of view, so when you owe them money it's shown as a positive value on the statement; a negative statement balance is the exceptional case where they owe you money.

When you reconcile a credit card account to a statement Quicken expects you'll enter the balance as shown on the statement. When you owe the bank money Quicken expects you'll enter a positive number since that what the statement has; Quicken then flips the sign to a negative on the next screen to match the account balance as shown in Quicken.