Thanks to both of you, appreciate your time to solve this problem!!! :)
This shape seems to be extremely hard to get accurate, and there are obviously some
issues regardless which technique I'm using so I don't really know what to do.
Is there maybe any other options in Rhino? Need to avoid poly modeling as much as possible..
was thinking it has some useful tools which might be handy.

Done some test renders.. post them later

Hi Persa, actually I don't think the problem here is needing other tools so much as figuring out a better strategy.

I was noticing the symmetry of that one tapered section, and I think I've figured out a better way to approach it.

For this new approach, I started by scaling up the corner piece from the bottom, this is scaled with the scaling origin at the bottom rectangle corner, I put in extensions using construction lines and got the corner where the construction lines intersected:



Then I moved that scaled piece directly up in Z, and dropped some lines at its ends straight down:



I think that's the key thing here - when surfacing, don't try to match the wider arc directly to the small one by a direct one-to-one connection. They are just too different in size and you will get pinching type problems if you try to match things of much different size. Things got a lot better when I tried matching the large arc to a bigger portion of the bottom including some of the straight pieces.


Then to help shape the surfacing, I added some rounded curves at the ends of the smaller arc piece. I actually made these by placing a second slightly differently scaled version of the arc in between them and then used Draw curve / Freeform / Through points to snap on to it:



Here is a different view of this curve framework:



Then I mirrored those pieces and joined the longer pieces together. Then these selected pieces become the profile curves for a sweep:



Then performing the 2-rail sweep:



I got better results with performing the sweep through all those sections all in one go. If you do it in a smaller section of just the curves in one corner, it will work but you will tend to get slight creasing where the sweep ends. By doing it all in one longer piece you ensure smoothness through the whole thing.

For the other pieces, I just did a straight extrusion up. I think you may need to adjust the other side in a slightly similar fashion, I didn't do that part here. Then after joining the extruded parts with the sweep parts you can then use Construct / Planar to cap the top and bottom to form a solid:



Then a fillet along the top edge (just select the top face and it will fillet all those top edges):



Another view:



That seems to be a lot closer - you could adjust some of the sizes, spacing, and curvature of those extra guides to tweak things further.

Model is attached as Back_part2.zip

Hope this gives you an idea on a different method!

- Michael