Travis, Please contact your local PRO associate to see if this item is available for special order in your area.
No Use a treated 4x4 when in contact with ground
I am not sure, but it is very heavy
To ensure you get a quality of lumber that meets your standards, it would be best to go pick it yourself from the store. I found that Home Depot lumber can vary greatly in quality from day to day. If you get there and the lumber sections were recently stocked, you have a better chance of finding good wood. At a minimum, you have many more pieces to select from so you can set aside pieces with cracks, bends, warps and knots that you don't want and pick the ones you do want to use. Other times, all that is left is lumber that has been picked over and may not be the quality you want.
Short answer: It could, if you proper hardare and joint techniques. Long answer: There are more variables that have to be considered to figure out how much weight this 4x4 will hold. The distance between the vertical columns (known as the span or beam span) plays a major factor along with the grade of the bolts securing the swing. If you secure the 4x4 to the floor joists, the grade of the bolts used and spacing of the bolts become factors along with joist material and design. You provided a static load capacity of 600lbs max. For safety you usually want to go 1.4 times that. So you need to support 840lbs. But a swing moves, thus it does not create a static load it creates a dynamic load. It can be mathmatically shown that a swing loading like this can produce 2 times the force of the static weight of the swinging object. So 600lbs becomes 1200lbs and you still need a factor for safety. So if we use 1.4 again 1200 x 1.4= design load of 1680 lbs. Now you have to consider how the weight is distributed over the span surface of the beam. For simplicity, lets assume you do not bolt the beam to the floor joist. Based on strength tables, a spruce 4x4 post has a bending strength of 10,200 pounds per square inch or psi. Assuming you use a 6 foot span, and using a flexural strength formula, this 4x4 (which is actually 3.5 x 3.5 after it's dried like this product you're asking about) can support 4049 pounds over the length of the span before it fails. Yellow pine is stronger at 14,500 psi bending strength and 5756 pounds of force over the 6 ft span before failing. So while it appears a 6ft span of this 4x4 beam can support that amount of weight, this exercise brings you back to how you plan to connect the swing and distribute the load over the beam with an assumed span of 6 feet. Remember this example doesn't factor in bolting the beam to the joist. Note: I am not a structural engineer. Nothing I publish here is meant to be instructional for how you decide to proceed with your project.
No it will rot
No, that means it was artificially dried using heat from a kiln furnace (as opposed to a slower drying process in temp and humidity controlled environment). Wood that isn't chemically treated should should only be used indoors.
I expect you could wrap it with aluminum. But since it's not treated I wouldn't trust that it would not ride over a length of time.