Thermometers for high temperature ovens

With thin cuts of meat (like chicken breast or a steak), you want to make sure you put the thermometer in to the thin side of the meat so that the whole thermometer goes in to the meat, rather than putting it in through the "top" so that only a little of the thermometer is in the meat. If you're putting it in the top, you can get wildly inaccurate temperatures. Also make sure that the thermometer isn't touching a pan, exposed to air, or touching a bone. Any of those can have a negative effect.

Also, you only need to cook chicken to 74C / 165F. That may be part of the problem in your case.

You may also want to check that your thermometer is accurate. You can stick it in boiling water to check that it reads 100C / 212F (assuming of course it goes up that high).

This is the cheapest chamber vacuum sealer that I've come across: VacMaster VP112. At $669, it's half the price of most other chamber sealers. Chamber models are far more "professional" than the stuff marketed to the home, and my understanding is that they are far more reliable.

The difference between a chamber model and a home model is that you place your food in a bag in a chamber. The entire chamber is used to create a vacuum, meaning that liquid won't leave the bag. A home sealer will also not go below atmospheric pressure while a chamber unit can get a much higher vacuum allowing for vacuum marination, compression or fruits, and better flavor penetration during sous vide, which may or may not hold any interest for you.

In terms of your specific questions:

1. My understanding is that these consistently make a good seal.
2. I can't speak to longevity, but these are the kind of units that restaurants use for sous vide prep, so I assume they take a beating in those environments.
3. Because it's in a vacuum chamber, liquid is no problem, and you can actually vacuum seal just liquid.
4. Major Fail. These things are big and heavy. The one I mentioned is 24" x 16" x 9" and weighs 53 lbs.
5. Shouldn't fail to seal. Not sure about waste if it does.
6. Cannot be used with a container, but why would you need to rather than using a bag?
7. Minor Fail. The model I listed is a little over your price range. Most other models of this type will be between $1,250 and $7,000.

The other consideration is chamber size. You obviously can't vacuum pack anything that's larger than the vacuum chamber. The one is listed has a chamber size of 12" x 11" x 5".