1. No, unfortunately your tamagotchi can't use the souvenirs at any life stage - they're just for collection purposes.
2. No although when you get all three souvenirs from the king (the ring, the cloak and the crown) your tama will receive the royal costume which is a usable item that can be worn. When your tamagotchi wears the royal costume it will receive a password which I think is only usable if you have the nintendo ds game? I heard it gives you a sofa on the ds game but you can't use it on your v3 tama.
3. Your new generation tamagotchi will keep the souvenirs you had to buy such as the travel souvenirs obtained from using travel tickets and the souvenirs received from the tamagotchi king. But the souvenirs collected or earned on tama town and the souvenirs from visiting your parents/grandparents will disappear and you will have to re-enter the passwords to get them back.
4. To get the first code from the king you have to donate 5000 gp in at least three istallments (e.g. 1000 + 2000 +2000). The next two codes are obtained after donating 12000 and 30,000 gp.
5. No, your tama will still age when sleeping and its hungry and happiness hearts won't drain which is useful. That's what i do with my tamas while I'm at school instead of pausing them.
Hope I helped!