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.
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.
I don't think why not. However, it should depend on the purpose. The design of the bracket is to have the short sides holding the lumber pieces and sustain the weight from the top layers. That is another reason the extended side facing down to provide extra support and forming a "strong math triangle" structure. If you flip it upside down, it would depend on your design and other bracket option might work better for your purpose.
Chris: they have a variety of models for different dimension lumber
This product is for 2x4 posts. RTC42 allows for 4x4s.
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.
Even though they are galvanized, I don't believe they would not last long with ground contact / weather
Search for the RTC42 model
no, I wished and I have suggested it in the past as well. They should stock them in the stores!!