Toilets in Arrochar and Tarbet

You have my sympathy - an interest close to my own heart.
I googled "Trossachs Toilets" and got quite a few hits, suggesting there are more than the ones you found.
Have a look.
Failing that, incontinence underwear is your friend, I'm afraid!

The ones at the Pier are certainly open.

Last time I was in Arrochar I was very lucky, and found the Church open, which had toilets, but that was exceptional.

They are not public, but you could ask nicely at the Arrochar Tea Rooms (or buy a snack).

Generally in these situations I find a pub and just walk in confidently. I've only ever been challenged once in decades of doing this (in Edinburgh on the Royal Mile, while I was living there, by an American lady!). I see there is a pub in Arrochar. Some pubs even participate in "community toilet schemes" which means they officially let you use them. But even if they don't, using them is generally not a problem. Good luck with your walk!

Having now done this trip, I may as well do a little write-up just in case anyone comes here looking for the same information in the future. The toilets at Tarbet Pier were open, and very nice and clean, and they shut at 5pm when we were there. They are in the same small building as a little cafe that sells tea and coffee, paninis, cakes and biscuits. We never investigated the Three Villages Cafe, so no news there.

We also found toilets at Succoth car park. These were chemical toilet portaloo type temporary structures, with no handwashing facilities, and were apparently only available during peak months (I think these were April to October?). Still useful though, even if you do need to bring hand sanitiser with you! And I don't seem to remember seeing any closing times on these - I imagine they are open 24/7 during the relevant months.

Thanks for circling back with your experience! I hope your walking went well too!