There is a way to get them in legally before they're 25, but it involves jumping through a few hoops and spending some money. After these cars turn 21 years old, they're OK to bring in as far as the EPA is concerned, but you'd need to use a Registered Importer and have the vehicles modified to meet US FMVSS so the NHTSA is happy, too. Because there's a "substantially similar" vehicle that was sold in the US, the parts exist for the modification. Totally legit. Only 21 years old. Bit of a PITA, though.
Yes, but, as above, only through a Registered Importer and only with FMVSS modifications as necessary. If you wait until 2022, you can bring it in no strings attached.
Same deal. If it's over 21, EPA doesn't care. NHTSA requires you use a Registered Importer, and modify the vehicle to meet US FMVSS.
As long as it was imported after it's 21st birthday, the EPA grants an exemption. No specs to meet as far as the Feds are concerned. You still have your local State or Municipality to deal with regarding emissions, though.
Yes, but if it doesn't have the US compliance sticker on it, it would still need to be brought in via a Registered Importer to be here legally.
Title and insurance does not mean a vehicle is in the USA legally. It is possible to smuggle a car past Customs and still get plates in some states. Insurance is happy to take your money but may balk at paying claims.