what hotels/rental homes accept an 18/19 year old to check-in and make a reservation?

No decent or reputable ones. Hotels and vacation rentals typically require you to be 21 and many require 25. You also have to have a credit card. A flea bag no tell motel that rents rooms by the hour probably would but that is about it.

You would have a hard time renting a standard rental as well. Many landlords will not rent to young 1st time renters. Those that will typically require a larger deposit or a cosigner.

Most rental vacation homes require someone over 25 on the lease.. Hotels I'm not sure. If renting long term (to live in) it's totally up to the owner though.

Depends on the situation to be honest. Going somewhere for a conference and need a hotel room, probably not a problem. Trying to rent a LOCAL hotel room for a party...big problem.

You'd need a credit card but otherwise you should be able to. (Same last name)

Probably not a rental car.

Any hotel, It is virtually impossible to rent a hotel room in the United States if you are under 18, the legal age of majority in all states. So if you are 18 its the legal age in almost all the states :)