I’m reluctant to ever say “always.” 
You just have to check each model to be sure, but as of this writing, July 2017, in general that is correct.
.2. If those with traveler wires a)don’t have radio transceivers, b) don’t talk to the Hub, how do they communicate back to the Master switch…some proprietary signal down the traveller wire?
Yes. It’s a pulse connection.
.3. Is it safe to assume, in a retrofit scenario (replacing hardwired mechanical 3 way switches), that the existing traveler wire is appropriate to connect to the replacement ZWave Addon switch Traveler terminal? I know there are many 3-way mechanical switch scenarios, depending on where the fixture is, the Line Power, each switch, etc.
It’s never safe to assume anything about electrical wiring. You should always map every segment of every circuit so you know exactly what’s going on. People do some crazy stuff when they don’t really understand how wiring works.
.4. In a upgrade of a mechanical 2-way (one switch) to 3-way scenario with ZWave switches, I’ve got to think the Linear switches are much simpler, no traveler wire to route between Master and Addon. Seems like this design is far superior from this standpoint, why don’t all manufacturers do it this way? Is there a down-side, besides $$$?
Different things work for different people. The primary benefit of the devices that use the physical traveler wires is that they will always work even if your home automation system fails completely. Some people prefer that design. And there is generally a dollar difference, as you mentioned. Certainly the ones that use a “virtual three-way” where there are no physical traveler wires give you more options in where to place the auxiliary, so that’s a benefit for that style.
.5. Since the Linear Addon switch does have a radio and does communicate to the Hub, I would guess that a Linear Addon might create the 3-way function with any brand of switch, no traveler wire required, right? I’ve read that Linear Addon works via Association, something I don’t understand.
There are two completely separate methods here. 
A) because SmartThings is a multiprotocol platform, it is indeed possible to use any two switches which can be controlled by the SmartThings hub to set up a virtual three-way. You can do this using the official smartlighting feature without needing anything else. It won’t matter what brand or model or protocol each switch is – – as long as each communicates with the hub when it is turned on and each can be controlled by the hub, you can set it up so that the hub receives a message from the first switch and then the hub tells the second switch to turn on. And the location of the two devices won’t matter, you could have a switch in your bedroom control a switch on your front porch. As long as both switches can talk to the hub, you’re fine.
B) completely separate from this, and completely independent of SmartThings, the Z wave protocol allows for what is called “direct association.” If the auxiliary is a Z wave device that supports association for Control of another device and the master is a Z wave switch within 1 hop of the auxiliary, then association allows you to set it up so that the auxiliary has permission to talk to the master switch directly without telling the hub what it has done.
The benefit of this is that it tends to be very fast. The drawback is that the hub doesn’t know that the events have happened, so the status in the App will tend to get out of sync with the real devices.
In addition, association only works between 2 zwave devices, only works if the device is specifically designed to be able to use Association for control of another device (and many are not), and only works if the two devices are pretty close together.
There are times when zwave association is useful, especially if you need really quick response, but most of the time if you are using SmartThings the benefits of creating your virtual three-way using the hub as a middleman will outweigh the benefits of direct association.
The following FAQ on GE three-way wiring may also be of interest: