Your math on the helix doesn't seem right. Let see if I understand correctly. Your radius is 350mm or 13.78". The distance between the layers of the helix is 114mm or 4.49"
%Grade is defined as rise (114) over run (2 x PI (3.1416) x radius(350)) x 100 = 114 / 2199 x 100 = 5.18%
Your radius is a bit on the tight side. You need to consider what engines and cars you plan to run. Large articulated steam engines and 80' or longer freight or passenger cars may cause some problems.
A grade over 5% will not be workable as an engine will pull very few if any cars up the grade particularly in conjunction with the 350mm radius curves. A grade of 2% may not give enough clearance for all rolling stock.
If you cut a slot on your uprights at 50mm spacing so that it catches the edge of your helix subroadbed you won't need the support under the roadbed. You will need to add additional uprights on the inside of the helix.
Hope this helps!
Waltzing Creek & Western Railroad