Short answer, yes, you can use a TS credit for CS.
Longer answer....we were a party of 2 on our May trip. We were able to make/change many ADRs the same day or a day ahead. I was travelling with a teen who changed her mind sometimes. Also, some TS restaurants don't take ADRs--Cap'n Jack's doesn't take any (long waits as the evening goes on). Planet Hollywood takes them only at least a day ahead--no same day ADRs. However, they said that they don't get busy till after 7. At 6, we were told there was a 20 minute wait, and we waited about 20 seconds!.
If you want to eat somewhere, ask! We always cancelled any ADRs that we weren't going to use....but some were only cancelled an hour ahead of the time. A cancellation = an opening for someone else.
If you can plan which park you will be at one day ahead, you can possibly get TS ADRs. Yes, it might be crowded during free dining, but the whole reason for free dining is that this is a less crowded season.