No need to buy the Metronidazole. If it's canker, the Carnidazole should work.
I would start with the Medistatin first since you saw mucus in the back of the throat, follow the directions on the label.
Since the meds are 5 days out, I recommend giving her an Epsom salt flush. The Epsom salt flush might even clear up the problem your hen is having.
Mix one teaspoon of Epsom salt into a cup of water (use measuring spoons.) Stir it well. No need to make a fresh batch daily.
Use a 10ml syringe without a needle and draw up 8ml and give it orally to your hen 2 times a day for 3 days.
Have someone hold the hen for you while you're administering the liquid. You can slowly dribble a little liquid on the side of her beak to drink. If she refuses to open her mouth or drinks too little, gently pull down on her wattles and her mouth will open. Then give her the liquid orally, no more than 1/4ml at a time and quickly let go of her wattles so she can swallow the liquid. Be careful NOT to give her too much liquid at once, she could aspirate, and dont give her more than 1/4ml at a time. Hopefully you'll be able to give her the full amount...8ml.
You should see improvement by the end of day two. Let us know how it goes.