Can you post a screenshot of what you're seeing with the taskbars and start button? You have taskbars on each monitor or no?
Just so I understand, you installed Start10 to use the Windows 10 Start menu? Windows 10 comes with a start menu already, so I'm not sure I follow.
http://i.imgur.com/NECJH4u.jpg Here is the picture of my desktop, with the Windows 10 style start menu. The task bar is only on the middle monitor. The reason I got Start10 was in the hope that it would move the start menu from the left monitor to the monitor which has the taskbar, but it does exactly the same as Windows 10, which is what you're seeing on the screen shot. Nothing else is wrong, it's just annoying.
Is your setup configured as eyefinity?
The default Win10 menu should appear on the monitor the start button that opened it is on, but if your monitors are being exposed as a single monitor then the menu may well be appearing far left on your 'monitor' as that's the OS default behaviour and completely expected as you only have the one monitor. This would also explain why you are missing those other multiple monitor settings in the taskbar properties.
Yes, it is configured as eyefinity, and the windows detects the monitor as a single one. However my problem with the start menu appears only when using the Windows 10 style(Start10 or original Windows 10 both have the problem). It worked in Windows 8.1, and it works with all other start menus from Start10.
Here's my current desktop with the modern style http://i.imgur.com/IXWxGir.jpg , it works, but you can see the Windows 10 style in the link above, which doesn't work.