If you use 5/8" plywood for shelving there shouldn't be a problem unless you are storing massive lead lined objects. This is a stout set up. Although the directions don't say it, you can place a reinforcing 2x4 under the middle of the shelves.
I recently used a primer on the metal beforeI painted with a latex paint. I will know as time goes on how durable it is.
Even though they are galvanized, I don't believe they would not last long with ground contact / weather
No. The whole point if this product is to support a 2X4 which will support the upper structure
I think that for a 4X6 post, you would need to remove too much metal for the 2X support to be effective.
What you need is RTC44. I'm looking for a local vendor, myself (scarce for So CA). We have a project where we want to frame 4 x 4 x 4s and the RTC44 is perfect. You can go to https://www.strongtie.com, navigate to Locations to find a "Factory & Training Center" and put in some calls. I think HD will help you get it, too; it's just a special order.
Oh, you could attach a 2x2 to this, but you won't be happy with the results. The exposed edges of the tie connector will maim someone, eventually.
This connector will accomodate 2x6 for the horizontal members, but not the vertical members. I'm unaware of a similar connector that will work with 2x6 vertical members without going to a structural column cap.
Garden beds would typically be built with lumber that has been pressure treated and is suitable for ground contact. Galvanized metal coatings react poorly with some of the chemicals used in process for pressure treating wood, so would not be recommended for this purpose.
The RTC44 will connect two 4x joists to a 4x4 post. Thank you!