"difficulty bomb" is more like a scarecrow specially because of its name. in fact they messed it up when they pushed it back a couple of times since this hard fork was supposed to happen a very long time ago. which proves it is not really a big deal. not to mention that Ethereum Classic (the original immutable chain) got rid of it about 9 months ago which means if anybody wanted they could simply pull the already tested change from ETC into ETH and continue mining in.
Oh absolutely. The original difficulty bomb was supposed to make Ethereum as it currently stands pretty much unusable by the start of 2017, to force the Casper update and moving from Proof of Work to Proof of Stake. It's been delayed before, and will be delayed again with Constantinople. And as you say, there's nothing to stop a side chain such as ETC from passing their own upgrade to delay or even remove it.
On another note, looks like a bug was discovered in EIP 1283, and so the whole of Constantinople has been delayed for at least a week or so while they figure it out:
https://medium.com/chainsecurity/constantinople-enables-new-reentrancy-attack-ace4088297d9