Are Coopers Tablets safe for undercarbed bottles?

By "safe" I mean, is there any chance they will stress the bottles and lead to bombs?

Due to inexperience and a faulty thermometer, I undercarbed a batch of bitter to the point where it's considerably more flat than I like. I want to open the bottles and drop a tablet in each, but I won't do it if the Coopers together with the existing (minimal) carbonation would be too much.

Meeeeh, it's probably best to err on the side of caution. Even if you do add the tabs, you'll be losing a lot of CO2 from the headspace (i.e. that nice "psssht" sound you get when you crack a bottle open) so you'd be wanting to add more like 2 tabs anyway, which seems even more dangerous

But what exactly do you mean by faulty thermometer undercarbed your batch?

Also, you may also not have given it enough time. The general consensus here is that the minimum amout of time you should give your bottles to carb/condition is 3 weeks stored at 70*F. This is also only for average gravity beers.... heavier beers will take longer. If after 3 weeks at 70*F you're still flat, then you should start to think of solutions.

After the 3 weeks @ 70*, cold condition in the fridge for about a week to allow the co2 to dissolve. I've had beers that were properly carbed after a week in the fridge that were flat if I just chilled them and drank immediately. Cold liquid absorbs co2 much better than room temp liquid.