Yes, miners can turn off the bomb, but that requires that they coordinate and determine a schedule for the hardfork that turns off the bomb (you need a hardfork to turn it off).
This scenario is definitely possible, but it requires a little more work than "one guy keeps running an old eth1 node".
How would they be on a fork, if the "original" dies because of the difficulty bomb? Since the fork that wasn't bombed would survive, it would be considered the original, wouldn't it?
How is it defined? Like somebody releases a new version with the bomb removed. Most miners adapt it. Why is it then a fork? Because the bomb is a major feature? Otherwise, wouldn't ever software upgrade be considered a fork?
If I understood the conversation from the last dev call correctly, they're talking about combining EIP-1559 and the difficulty bomb fix into the same fork specifically to avoid this