As it stands today 6/9/2024:
1. After 5 years as a Green Card holder.
2. 5 Years as a Green Card holder & at least 65 years old before qualifying for Medicare. Also, normally 40 quarters (10 years) of work credit by immigrant or spouse for free Part A. They might be able to buy in at 65+ years old....if they have been Green card holders for 5 years. In 2023, the buy-in cost for qualifying people was roughly $670 per month for Parts A & B. (exceptions may apply). I wouldn't even guess as to what the healthcare system will look like in 20 years.
3. Yes
4. About 20+ years. Visa numbers are just now available for those who have priority date (I-130 submission) of Jul 2007.