Some quotes in this context:

1) "Tex" has a M track helix:



"H0" had an idea about catenary in a helix:



Video of a double track helix made with M track:

I'm planning/building a helix based on C-track so I suspect the same geometry considerations would apply for M-track.
For the rise, yes, any added height, between one base of the track and the base of the track after 1 revolution increases the grade.

So raised pantographs and catenary towers do require a revolution make a bigger rise. If you plan to operate with the pantographs up (contacting and riding off of the catenary), attaching some sliding surface, flex track or even blank wire to slide over, is a good idea, as you can mount that directly to the bottom surface of the loop above. The only trick is the trasitions at the entry/exit where you need to gradually slope the contact surface to/from the height of the catenary posts. If your powering via catenary, that would be the best option. If your only using the catenary cosmetically and artificially locking the height of the pantographs so they don't fully extend, then you have pretty much the same constraints and just need to make a safe exit from the non-catenary helix/tunnel onto the wires that then become the visible catenary space - to prevent any pantograph tangling.

Märklin locomotives tend (in general) to have much better climbing characteristics than say the bulk of DCC locomotives, because Märklin use traction tires more and metal body and chassis locos often weigh more, all providing better traction. I'm 'planning' on doing an experiment when I start building in 2019 to see how high I need to crank the grade in the helix before the train cannot smoothly get up it anymore. (via the threaded rod assembly method). A key factor of the maximum grade isn't just the loco but also the train - the cars and their weight and length. In my model world, I'll be running short trains pretty much exclusively, so the loco only needs to pull 3-4 passenger coaches or 6-8 freight cars. If I was wanting to pull prototypical 9-12 passenger coaches or 40 car freight trains then I'd likely need a much larger (oval) helix than I'm planning.

My plan is an R1/R2 C-Track helix. I've seen many youtube videos of such helix working fine, so I'm not expecting any problems, but, and to speak to Tex' good advice, I've planned my 2 track helix so that the climbing side is on the outside (R2) track - thanks to Germany being right hand drive this isn't an issue, I just go up counterclockwise. The R1 inside radius is the downhill side, thus the concerns over R1 climb grade go away. So my added advice is: design so that the climbing side of the helix is the outside track - so if your Germany your helix goes up counterclockwise, and if your are the UK for example, it goes up clockwise.

The general wisdom is about 10cm of space is needed per turn to allow for the space for track, loco, etc. The thinner the wood, the less space you need (but the more it may sag unless reinforced). If one builds a carbon fiber track base, that would be the thinnest I imagine. Good plywood is the minimum - stay away from any particle/strandboard/MDF solutions as they will sag over time from room moisture. (see Everard Junction's older layout)

Thin acoustic buffer between track and helix bed also cuts down on the grade - if the helix is to be put inside an enclosure (eg mountain scenery ) where the sound is kept inside, then adding cork/insulation may not even be necessary - screw the track to the wood directly.

When building a helix, plan in a way for you to get yourself inside the center for problem resolution and possible occasional track cleaning work; maybe plan in some LED lighting and a camera if its all to be enclosed inside landscape.

Here is an example...