My uncle (blacksmith/inventor) is pretty certain that step 1 (cutting the pipe) was a mistake, as he had a solution (which he e-mailed to BP, which of course ignored it) which involved the pipe (and a clamp).
OTOH he doesn't have data on the exact pressures and forces involved, so BP probably had far better information on which to act. (and it's far easier to be an armchair engineer!)
The pressure at 5000 ft under the ocean is roughly 2000psi relative to atmospheric pressure. 3500psi is the difference between the oil pressure and the pressure at the sea floor.
Maybe BP should have crowd sourced this problem. Publish all of the data that its engineers have access to. Maybe even offer a prize for a working solution.
The pressure differential between the oil and the water is pretty low (20k barrels/day isn't that much for a pipe that large) so crimping wouldn't require much more energy than bending a pipe without any flow.
However, I don't know if there are ROV attachments that are strong enough and whether there was the required access.
The other problem is that a crimp probably won't result in a complete seal. I don't know if there was room for multiple crimps or their cumulative effect.
On the other hand, crimps would have reduced the flow while the relief well is being drilled.
The other problem with crimps is that they may make it impossible to do other things.
I can't believe the DIY'ers really think BP is filing their submissions anywhere but the "circular file".
However sometimes when I watch the "oil cam" I secretly wish to see someone in some kind of DIY "iron man" deep-diving outfit suddenly appear and walk over to the pipe and start fixing it themselves, lol.
OTOH he doesn't have data on the exact pressures and forces involved, so BP probably had far better information on which to act. (and it's far easier to be an armchair engineer!)