Smaller curves (your 3.5 diameters) are going to be hardest. I did a quilt with circles, and decided to redesign it using nothing smaller than a 5 or 6 inch diameter circle. but that was just me. Partial circles/arcs/curves at that smaller size should be more doable than full circles. Starch was definitely my friend, as it kept things from falling apart too bad with frequent seam ripping, otherwise, like the others said, match lots of points, whether that be all the apexes or half way/quarter way marks (not sure if you are doing long wiggly curves or quarter circles or what. Good luck, it'll be worth the effort when done!

I have the Curve Master but could never get the hang of it , so I continue to struggle with curves .

I find the half and quarters of the two pieces and mark or finger press small hash marks. It gives me more reference points. For small circles, I have had good luck with gluing basting and then top stitching.

Or you can use light weight fusible interfacing and then applique the inner part to the outer part.

I'm not sure curves are meant to be quick and easy. They are meant to be something you take your time doing. I would find it very difficult to write a pattern without having my own expertise up to par.

I pinch and mark the center of both pieces, pin that and then the ends. I pin anyway, and I pin a lot on curves filling in by quarters from the middle and sides. Generally it curls up in a ball as I sew but I can't swear on which side I put on which way.

Although I generally press open, curves are the only time I press to the side.

I do everything I can to avoid handwork but I'd say that curves like on a drunkard's path are probably best done by hand below maybe about a 4" final size. Sure you can (and I have) machine pieced smaller ones but there are diminishing returns.

Melon shaped arcs aren't much of an issue for me, and grandmother's fan sized ones are relatively easy as well. However, this past year I used what I call the "Eleanor Burns Technique" using rickrack for the curved edge and I really liked it, it added texture and interest to my project and was very easy to use. I think it starts around 18-19 minutes into this video:
https://www.youtube.com/watch?v=Ktzb2wMMwzc

Like GingerK, I find using a glue stick (washable) works really well. I run a line of glue along both pieces. Then I attach the ends and the middles; once that's done, I ease in the rest so the whole curve is glued. If you are using good 1/4 inch templates, it will fit together perfectly, and then the sewing is easy.

I use a lot of pins and clip carefully to make my curves.

I think of "concave" as a nipped in waistline - and "convex" as a padded bra.

Could the pattern be somewhat modified to make the curves "gentler"

This is exactly how I do it, and it works beautifully. I agree, you don't need the Curve Master. It's the same technique the inventor of the Curve Master teaches to use her product, but you can achieve the same results with your regular foot.

I also agree with this. Years ago I started an apple core quilt, the pieces went together just fine but I ran into major problems when I started to assemble the rows. Went online and researched, found that concave on bottom worked beautifully. But when I realized I would have to either assemble the rows by hand, or, if sewing by machine, take the quilt out of my machine and flip it after every block, I wadded it up and tossed it in a corner. :p