the whole plot twist was based on that, and either they were so confident they would agree to it or it was staged, but if theyd said no, then i dont know how ch5 would have gone on to fill the big surprise and all entailed, theyd have been stuffed!
also i didnt get that j&f walked into diary room , and what if anyone else had been walking there, coming in, going out and seen them or wanted to use the room when they were in it?
was it all arranged and they really all were in on it before it startd and we are being taken for mugs?
and this is not a tread where its an excuse to bash anyone, it is more at the reality of what ch5 would have done.

Well that goes for every secret task the show has ever done

well same thing then, when they have had such a big build up for days, i really dont know how theyd just then say "oh sorry, its not happening now, never mind!"

They read the script before they went in the house.

i'm sure they are contractually obliged.

I imagine that the contracts require them to stay until the show ends or they are voted out. Regardless of what they thought, they hadn't been voted out so they'd have forfeited money if they hadn't gone in. (If anything it's the civilian version which is the risk.)

There's always more than one door into the diary room (they need a staff entrance apart from anything else for tasks, medics, etc). They also quite regularly keep HMs wanting to use the DR waiting outside the door until they're prepared to see them. (Edit: Or did they come down the corridor? Still, the other HMs were probably being kept on the sofas.)

It's always worth being suspicious of whether things are as Ch 5/Endemol are showing them, but I'm not sure it needs special explanation in this case.

If they didn't go it, they hadn't been evicted so they technically would have walked, thus losing their fee?

Emma did ask them if they wanted to go in the secret room so I think they could have said no

Docked their pay and got a bargain. They were never going to say no.