I appreciate everyone's input and ideas on this.
I will have to source the relay locally as the exhibition is happening quite soon and deliveries to islands (Malta in my case) are a gamble at the moment.
This is what I found which would seem to work for controlling both the forward and backward function, am I right?
I think I understood my mistake. Sorry if I'm not making sense, this is one of my first actual project with Arduino. If I wire the relay in parallel to the existing remote control circuit board, it means that I will not have to disconnect anything (which I did not understand until now). My technology lessons in secondary school (which I loved every bit of) are coming back thanks to this process and your precious help.
Eventually I'd like to do something like this for them, yes! I'll start with making the simple forward automation and will eventually develop the project and share some pictures and the code on this thread.
I did think that being able to keep track of where it is at would allow for custom time for each slide or any other creative use of this custom remote control system. I keep it in mind.