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They should be fine, though it's dependent on what type of seed. You can test the viability of seeds in one of two ways: Slightly dampen a paper towel and place the sample seeds on it. Fold the barely damp paper towel it in half over the seeds. Enclose in plastic wrap or place inside a sealed plastic bag so it will stay damp. Label the package with seed name and date. Set the package in a relatively warm place (70 to 75 degrees) such as the top of your refrigerator or on a high shelf. Do not put it in direct sun. OR you can simply float them in water. If they sink, they're still good and if they float, toss them. This article should help as well: https://www.gardeningknowhow.com/garden-how-to/seeds/planting-old-seeds.htm

This article will help with that: https://www.gardeningknowhow.com/garden-how-to/seeds/soaking-seeds.htm

I would consider it to be effective. It is best to water young plants from below and these kinds of mats make this much easier to accomplish.

Yes, you can plant non-hybrid seeds where you planted hybrid seeds the year before. Hybrid seeds do not affect the soil.

This article should help: https://www.gardeningknowhow.com/garden-how-to/seeds/planting-old-seeds.htm

The soil temp for seed germination is pretty much the same across the board regardless of location. This chart should help: http://www.heirloomseeds.com/germination.html This article will help with soil temperature: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/determining-soil-temperature.htm