Transportation between Art Islands

I am visiting the Art Islands on October 7th (Monday) and October 8th (Tuesday). On October 6th I am staying in Okayama. October 7th staying in Naoshima. October 8th staying in Osaka.

On October 7th my plan is to take ferry to Naoshima drop off my travel back pack in a locker or at my hotel which is near Miyanoura port then head over to Inujima and Teshima for the day. As it is a Monday, the museums in Naoshima are closed, but the museums in Inujima and Teshima are open. I am under the impression that Inujima takes 2-3 hours to see everything while Teshima takes a half day. However, the Benesse art website indicates that there are only three ferries per day one in the morning, one around midday (~13:00) and one in the afternoon (last from Inujima 15:47 last from Teshima 16:12).

I think I can probably see everything on Inujima and be selective about what I see on Teshima to make sure I catch either the midday ferry to Inujima or the last ferry to Naoshima. My question is 1: Why does the last ferry run almost an hour before the museums close? 2: What happens if you miss the last ferry? 3: Are there private water taxis one can take and if so how expensive are they?

Since the ferries between the islands don't run at all on Tuesday it would be an absolute nightmare to be stuck on the island Monday night and through Tuesday into Wednesday morning.

There is a Marine Taxi service offered by Kawanishi Marine Service. Between Inujima and Naoshima, you can choose the port (prices are 19,000 yen one way regardless of the port):
https://www.kawanishi-marine.com/marine-taxi/inujima/196/

You can also depart from Teshima to Naoshima:
https://www.kawanishi-marine.com/marine-taxi/teshima/187/

You should make a reservation as early as possible, especially if it's during the art festival.

Every time I have looked at those two minor places the ferry timetables dictate either 10 minutes or a huge wait with nothing to do. (and they have a lot less to see/do than Naoshima.)
If what you wan to see is closed on Naoshima, why go there. (BTW - I haven't hit the art house project being closed - just lucky - was last there a couple of months ago.) Checking the web - Monday is not a closed day for something like Benesse Museum (although small, over-priced and not that interesting.) There are out-side attractions.

The museums on Naoshima are open on Tuesday, but not Monday with the exception of the Benesse House. So I will see Inujima/ Teshima on Monday and Naoshima on Tuesday before heading to Osaka. I will also see any outdoor stuff on Naoshima on Monday when I return.

I am only staying in Osaka overnight before heading to Nara/ Kyoto the next day. As I keep trying to tell people on this forum Osaka is pretty boring. I am excited about TeamLab botanical though … and I can go for a brief walk on Dotunbori, but my impression is that Dotunbori is just Times Square in Osaka.