Do I have any control from which of my yellow, non-empty addresses the send BC amount is taken?
Yes, electrum allows to select which address(es) you want to spend from.
To do so, go to "addresses" tab and right-click on any of addresses you want to spend from and select "Spend".
Note that you can use "Shift" and "Ctrl" buttons to select multiple addresses.
You can also select "Freeze" to tell electrum to not use that address for your transaction.