The coolant leak I can't find the source of

Yep. The 90° connector on the heater tube assembly under the intake has most likely cracked due to age. Mine did bout 5 years ago. There is a drain hole at the back of the intake valley to allow water/coolant to drain out. (See pic). It's a much easier repair to remove the intake manifold. If you're on a budget, you can buy a Motorcraft or Dorman connector, remove the crimp clamp like in the video, and clamp on the replacement. Do not cut the hose. It will be too short after. But most likely, the heater hoses are due for replacement anyway.

Here's the Motorcraft/Dorman part:

Here's the heater tube assembly:

and heater hoses:

You'll also need to replace the O-rings on the nipple/tube that the metal heater tube presses on to on the back of the water pump.(See pic). The nipple is pressed in to the engine block behind the water pump. Once you have the heater tube assembly off, remove the 2 O-rings, and match them up at your local hardware store or you can buy the Dorman nipple and take the O-rings off and put them on your existing nipple.

Removing the intake manifold will most likely also require new intake gaskets & injector O-rings:

A pair of remote hose clamp pliers will also come in handy.

And Motorcraft Coolant (I prefer the 50/50 pre-mixed):