<human>: Give me a shortened version of the text

So I have been using the standard flight search tools (Kayak, google flights, hipmunk , etc) and I have been having trouble meeting the specific parameters that I need. Here's the story:

I am deepply and passionately in love with a girl. She is on a solo hike through england until june 15th. She arrives in Edinburgh on the 15th for 5 days, and her flight back to where Toronto(where we both live) is on the 20th of june. Once she gets back she has about 10 days before leaving to go out east for the summer, potentially not moving back(whole other story). 

We thought it would be amazing to spend that 5 days in edinburgh together, and to make it more worth while I would go a few days earlier to explore the city by myself, probably friday the 12th or thursday the 11th. 

I don't care how I get there, how painful it is or long for the flight to get there. I just want cheap. BUT the return flight, I want to be on her flight in her class so we can sit together. Herein lies the problem. I know her flight details for the return flight, and I want to just input that exact flight to one of these engines, and say THAT is my return flight, find me the best(cheapest) flight there. None seem to be able to pick a specific flight, just search based on price. 

I know i am crazy, please don't try and talk me out of it.
<bot>: I need a search engine that lets me pick a specific return flight, and then just search based on price and a general initial flight out with a few days leeway.