Merkel isn't on shaky ground. In Germany a healthy majority even at this point thinks the benefits outweigh the disadvantages; the numbers even increased right before and after the UK referendum.
You also shouldn't underestimate that many Germans are actually frustrated with the cherry-picking of the UK and other countries - even now. Also, many are frustrated because Cameron blocked better controls of the finance industry, eradicating tax havens or because he blocked better working rights. So other that the prejudice might suggest, the frustration isn't limited to red tape or the bail out for the Greek: It's a very mixed bag, and the UK is part of it.
I'm not suggesting that my environment is representative of Germans. However, I haven't met a single person since Friday, including workers from the car industry, who doesn't take another stance than "in is in and out is out" - and expect that from Merkel to be implenented. This includes the car industry workers who are well aware that cars are exported to the UK.
My friends in the finance sector are actually eagerly looking forward to thousands of jobs moving from the UK to Frankfurt.
My friends in the pharma industry are looking forward to the European Medicines Agency being pulled from the UK, and although it's not determined yet where it'll move, it will be to a country that isn't likely to leave the EU soon. Hence Germany should have a good chance they tell me, and apparently a couple of American and Asian pharma companies have communicated that they'll reduce staff in the UK and hire elsewhere, moving their EU HQs staff close to the new location of the agency.
Of course one fine day there will be a deal. It just won't be better than what has been on the table right now.