Kudos to you! I was so glad to read that you are being smart and keeping your trip to the western part of the U.S. Very frequently we hear from folks who want to drive all over the U.S. in a very short amount of time. But given that you are from Australia (another large country) and have experience with long distance drives, it looks like you will be sensible about your planning.
The itinerary you've posted sounds good. You've got many of the main sights in southern CA in there. I would suggesting taking time to see some of the top attractions in LA - the walk of fame, hollywood sign, la brea tar pits, rodeo drive.
Regarding taking a rental car into Canada, it should not be a problem. This is the policy from Alamo Rentals:
Rentals originating in the United States (USA) can be driven throughout the USA and Canada. Vehicles cannot be driven into Mexico. *Please note: Canadian citizens cannot rent a car in the US and drive it into Canada.
Notice that you cannot take it to Tijuana. You can check the other major rental car companies by following the links from our Car Rental Tips page. www.usatourist.com/english/tips/carrental.html
To visit your friend in Omaha, you are right. Flying probably would be best as driving will add on at least 2 additional days each way depending upon your route.
I am not certain what you mean about the hire cars from A to B being free or cheap. I am guessing that you are referring to a drive away or auto transport. This is where you drive a car for an individual or a company from point A to point B in X number of days. This is not very common and only comes to mind because someone else just posted a question about this to our Forums. You can read that post and my answer here: http://community.usatourist.com/forums/t/921.aspx
And kudos again! You are right about the buying/selling a car plan. We state on USATourist to not do this because of the difficulty and high cost with insuring and registering the vehicle without a permanent US address. Folks who read our page on this don't want to beleive us and will write us insisting they want to do it and they want us to tell them how they can cut through all the red tape. Thanks for sparing me having to disappoint yet another traveler with my discouragements! 
Many hotels in the U.S. do offer a free continental breakfast. This means a buffet of coffee, cereals, fruit (apples and oranges), and baked items like muffins or danishes. The quality can vary. You may get lucky and find hotels that do provide hot items like scrambled eggs and bacon. But for the most part, you can expect this to be very simple. Other hotels will offer a nicer breakfast for a fee.
Of course the best accommodations and breakfast can be found at a B&B. However in the U.S. these can be quite expensive at $200 on up per night.
Motel 6 is an economy hotel chain. They are very popular among the cross-country big-rig drivers. I stayed in one of these years ago and it was neither clean nor comfortable, but it was cheap! It was a long time ago, so perhaps they have changed. They do certainly advertise a lot on U.S. tv, so perhaps they've made some improvements over the years.
I would consider Super 8 to be a small step above a Motel 6.
Generally you can be fairly certain that you will have a clean room if you stay in one of the other, slightly higher cost chains like Comfort Inn, Holiday Inn or Best Western. You can view a listing of these chains on our Budget Hotels page www.usatourist.com/english/tips/budgethotels.html. Our moderate hotels page lists the chains that cost a bit more but aren't luxury level. www.usatourist.com/english/tips/moderatehotels.html.
Some hotel properties (particularly on the budget chain level) are privately owned. So the cleanliness and quality can vary, depending upon how well the owner manages the place. Also, cheaper hotels are generally less likely to offer anything other than coffee in the morning. I recommend using our hotel search engine. www.usatourist.com/english/reservations/reservations-hotels.html. You can read reviews from past guests to get a better idea of how clean, comfortable and quiet that hotel is. And in the property description you can see of a breakfast is offered. It is possible that a hotel will offer breakfast and not list it as an amenity. But in most cases they will list everything that they feature. And knowing if you will be getting a free breakfast at the slightly more costly hotel than at the cheaper, and possibly less nice hotel can be a decision maker.
Good luck with your trip!
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