Watched this recently and was pleasantly surprised. I found it pretty gripping, and the 70s tech actually plays right into that. The communication with the computer via text is really well handled to build tension.
They planned the whole idea out in a grassy area near the parking lot... why couldn't they just keep meeting out there to conspire?
I think it makes perfect sense. How else could he send/get information unless he had a plan to convince Colossus to not observe him.
My big issue with this whole thing is that they actually believed Colossus wouldn't listen in because it said so.
Yes, this was the other big plot hole.