>If I limit my use of OCX 5 to Imagelist, ToolBar, Treeview and Listview, would I (or my users) have anything to worry about?

First that I found is that 5.0 toolbar has no dropdown buttons. I think this is the main feature leak you have to worry about. Another problem is that 5.0 and 6.0 use different ocx files and you need to distribute comctl32.ocx too.

>I don't understand why Visual FoxPro not a Winform engine.

Because all .NET languages have common base. It means that .NET converts source code to common language before compilation. This allows for example to use classes coded in VB.NET from C# app w/o any intermediate interface.
VFP is still not a .NET language and probably cannot be .NET because its conception is not compatible. I remember something about this was in December letter from Ken Levy.
But I believe that VFP can provide an interface to .NET classes in future.

>I used a Winform Xbase (released by same people that offer Delphi) about 10 years ago. It was flushed out by Visual FoxPro, but still enjoys a small market in europe.
There no link between Winform Xbase and WinForms.

Now I found a way to provide ">>" button and even "custom" customization for MS Toolbar 6. Now I working on release version. (My own uses my internal classes). So please wait.