Ok, I worked on it and my conclusion is, the best way to handle this is to use the ControlAdded and ControlRemoved events of the splitcontainer to check which control is currently added/removed and merge/unmerge the toolstrip of that control w.r.t. the Home form toolstrip. Just write a global method to merge/unmerge the toolstrips with a parameter for the name of the control (the control that'll be added/removed in the splitcontainer). In the ControlAdded and ControlRemoved events, call the method with the child control as argument. Refactoring the subforms that I mentioned in the OP to UserControls improved performance but in certain cases they don't fulfill my requirement e.g. when Subform1 is opened in the splitcontainer and Subform3 is opened immediately from it without focusing on any other control of SubForm1 (suppose by clicking a toolstripbutton on SubForm1), unmerging doesn't occur as events like LostFocus, Leave etc are not fired. This creates ambiguity in the menus. So I'll go for the Splitcontainer ControlAdded/ControlRemoved events. But instead of opening forms as child controls I'll henceforth use UserControls.
I'll drop a small question at the end. How'll I close the UserControl from the SplitContainer panel? Till now I was using:
Code:
frmSubForm1.Close()
frmSubForm1=Nothing
But UserControls don't have any Close() method.

Fell free to continue this thread further with your suggestions if needed.