A decent, long-stem dial thermometer is actually a robust instrument, but it's prone to going out of calibration. As long as you have a good laboratory-standard thermometer that you can calibrate your 'working' dial thermometer with, you will be in good shape.
No need to buy a Thermopen since they are horrifically overpriced and not worth it. However, if you are interested in electronic thermometers, Thermoworks (the same folks that make the Thermopen) have great handheld units that are very inexpensive and accurate. You won't feel like you've been screwed.
On to the mash temp. Yes, 156F is too high for any higher gravity beer. I find that 152F is good for most medium and high gravity beers and I only boost the temp if brewing small beers where I need a higher finishing gravity. Underattenuation is the primary fault I find in homebrews because the brewers think that 'chewy' beer is better. (Its not) An appropriate level of fermentability and attenuation is what makes a great beer. If you need the beer sweeter, reduce the bittering, not reduce the fermentability.