1. nowadays, perhaps not much difference. What matters is volume of film coming in and freshness of chemicals. Some labs like Dwaynes Photo or North Coast Photo have high volume, fresh chems, and excellent quality. But so does my local Costco, at a fraction of the price for dev only. Many places now sadly have lowvolumes, old chems, poor quality.
2. I started out using Kodak 7 bath E6, ran thru maybe 5-6 5L kits before it was discontinued. Since then, switched to Tetenal, and would never switch back! Same high quality, vibrant colors, longevity, and no grain increase. Simpler and much easier to mix up and use too.
3. If you get the Tetenal E6 kit, shelf life once opened is a minimum of 8 weeks. Could be longer, if you use a protective spray like Protectan or Dust Off (what I use now, as Protectan is not available in US). I have read posts claiming 6 months, but cant verify. Dont know about the C41.