Thank you for your post.
I see that you had already reported this previously to FileMaker back in June 2008, and also to this forum back in August 2009 (Pasting bug in FileMaker Pro 10), where I did forward your comments to Development and Software Quality Assurance (Testing). I know reporting doesn't fix the issue, but we are aware of the issue. As it currently stands, custom menus only support one type of shortcut.
This may be a design issue/limitation, so to cover all bases, I would also urge you to enter this post into our Feature Suggestion web form at:
The information there is read by Development and Product Management, so a different set of eyes will see this.
Although I could easily copy your post and paste it into the web form, there are additional questions asked that only you can answer.
I've always considered ctrl-c and ctrl-v the "standard windows shortcuts for copy and paste". They're also more conveniently placed than the insert button.
Those are standard for Mac and are what's shown as standard in many Windows apps. Nonetheless, I've found CTRL-Insert and Shift-Insert are widely used on the Windows side. Actually, I used to think I was the only one...until my customers started complaining to me after the advent of FM's custom menus. I guess the placement depends on your keyboard and your mouse. I keep my mouse on the left so that I have my right hand free to make notes while developing. So I highlight text with my mouse hand and then like to use my right hand to hit the keyboard for copy/paste. CTRL-C is awkward to use with the right hand, but CTRL-Insert is very easy. And you'll find that this key combo works with every single application in the world (that I've ever used) -- even with FileMaker's regular dialogs (define calc, for example) and field values...except when you've turned on a non-customized custom menu.