Thank you for your detailed feedback — I'm really sorry about the problems you encountered.

Regarding keyboard shortcuts: it is possible to assign a shortcut to search for the next occurrence and another shortcut for the previous occurrence (although unfortunately "Enter" cannot be assigned as a shortcut key). To set this up, click the puzzle piece icon to open the Extensions page, then click the gear icon to open the tools menu, and choose "Manage extension shortcuts."

About your comment on matching the default search behavior more closely: I took it very seriously and made two significant changes.
First, in the search window, you can now click the "eye" icon and choose the half-open eye mode: highlights will only be visible when the search window is open, similar to the default search bar behavior.
Second, there is now a new option in the display settings to choose what should be shown when you reopen the search window — including an option to automatically select all the previous search content, allowing you to instantly replace it by typing a new word.

As for the highlight colors: all highlight colors are fully customizable through the display settings (just click the gear icon in the Multi Find window).

Concerning the random arrows appearing on the page: I have fixed a bug that caused unexpected scrollbar arrows, so hopefully it should now work much better. However, please note that scrollbars (and their arrows) can appear not only on the main page but also inside any iframe embedded in the page.

Finally, regarding the extension completely stopping working: I don't have a clear explanation, and I am surprised that uninstalling and reinstalling didn't solve it. Perhaps cleaning the browser's cache could help?

Again, thank you very much for your feedback: it helps me improving Multi Find!