I am wondering if my customers will still be able to use the In-Browser editing tool that muse provides, once the app is no longer supported. Basic changes, text, images.
Muse is not being updated anymore. I think the last update rolled out in April. So if your server supports In Browser Editing currently, yes, it will most likely keep working but for how long is anybody's guess. Personally, I would not invest any more time or energy in Muse. At least for new projects, you should be looking at other long term solutions like WordPress & the new Gutenberg Editor as one example.
maybe... it really depends on where you host the site because Muse will not be getting any new updates and I know at least 3 systems have changed their layout so ibe no longer works for them