Shimanami kaido, same day rental

Hi
Been curious about Shimanami kaido but due to shortage of time I can probably only try it for one day, possibly going half way and ferry back to Onomichi.

Is same day rental possible in end of February?

Since I stay in Osaka and carry lugguage I need to make sure I don't go there in vain.

I thought of finding a hotel in Okayama or Fukuyama as it is closer to Onomichi.

Yes, 1 day rental is possible. However Shimanami Kaido is very popular (probably not that much in February though). So I would strongly suggest you reserve a bicycle online before you go.
There are several places that offer such service and they are easy enough to find online.

As it seems the rental options mentioned on Japan guide won't accept reservation too close to the day.

I was looking for options around half way and seems some are available, as it will rain later in the week it seems, it would be best to go not too late.

How does it work if I can't get back to closing hours, will it start another rental day or do I get a penalty fee?