I cannot reproduce the issue, it's fine entering a value from the keyboard into layer translation and scale.

Nevertheless, I'm fine with changing the callback signature. But I cannot find any reference to it accepting int as return?

According to gtkd.org it should have a void return type?

Thank you for comment.

Nevertheless, I'm fine with changing the callback signature. But I cannot find any reference to it accepting int as return?

According to gtkd.org it should have a void return type?

A certain source file on the web used int type, so I copied it.
However, in response to your comment, I have investigated again.
And I found some samples ([ _focus_in_cb ], [ GUI_ed1_cb1, GUI_ed1_cb2, GUI_ed1_cb3 ], [ Drag and drop example ]) which callbacks do not return void.

Indeed, the link you posted above also said the following:

This doesn't mean that all callback functions must take no parameters and return void.
The required signature of a callback function is determined by the context in which is used (e.g. the signal to which it is connected).

This means that the signature of the callback function seems to be context dependent.
In our case, the "focus-in-event" signal handler returns gboolean type. Therefore, it seems to me that the relevant code should have been modified as follows

gboolean
focus_in_event_callback (int *widget_num)
{
	focused_widget_num = *widget_num;
	return FALSE;
}

Thank you @kitanokitsune for all your work! It would be great if we can merge our efforts :-)