Christine,
I have just looked at Photozone review of your 18-55. That suggests best performance is at 35mm setting. If you reverse that onto the 75-300 at that setting and then use the zoom function of the 70-300 to get focus. Nightmare to focus yes, as by now as you have discovered dof is non existant and even finding the subject in the viewfinder is hard enough. The uncropped image suggests you were closer to the 70mm end. Very similar to my try with the 105mm. I found that had gone by the time 200mm was used. Do you still have an old 50mm that you can reverse on the 70-300 as f2.8 will give more light for focussing.
Well done for giving it a go.
I am still playing with various combos for ease of use, and todays results will follow in a minute.
Don