Yes, but probably no pre-made bombkit from the internet .

I strongly doubt the availability of such things online, but I don´t to be difficult about it, it is fun to speculate..

Okay, as a fanfiction writer I have Googled some very odd things in the last couple of years, bomb making among them (and that isn't anywhere near the oddest) and making bombs that works is frighteningly easy even without much technical skill. If you do have even some technical skills it's even easier.
All in all, I can only conclude that the terrorist threat isn't nearly all it's cracked up to be. Because if there really were all those people out there dead set on blowing us up, it would happen a lot more often than it does. I this I include both terrorist plots revealed by intelligence agencies as well as those who are actually carried out.

I agree with Lily. And John's reaction? He just resigned. He realised that he likes Sherlock and that he had to either immediatelly kill him or accept as he was. So he began to laugh, too, but it was not too sincere laughter.

As to the off-switch - I would expect some way how to swich off the bomb though not so visible. And also some manual on-switch (does such a word exist?).  When somebody switches the bomb manually he must have time to escape, it might be complicated (and useless) to have two different systems for both manual and remote control, so that is why there was always delay. 

Maybe it is far from reality but explanation good enough for me to accept the scene.