Best Of

I would recommend working on more complex sketches with "Show constraints" turned on.

The two ends of the R46 arc looked like they want to be tangent, but they aren't. Even after fixing those and reworking the constraints a bunch of times, I couldn't find the unconstrained geometry. The sketch solver gets weird sometimes with things that were created from offsets and fillets. I tried getting rid of the offsets and adding reference lines to make all the "offsets" equal manually. I was still baffled why this was unconstrained.

I went back to a basic principle, which is to put your layout in a separate sketch and then hang all the "offsets" and tangents off of that. In doing so, I found a bunch of hidden (covered by other sketch entities) points which were likely created during offsetting and trimming to create fillets. I deleted those, and was able to make everything constrained. I'm sure that the layout sketch and your front view could be rolled back into a single sketch, but separating them is probably better practice to keep things more manageable.

https://cad.onshape.com/documents/646f8abfd3573a2de3c6f26d/w/df652c88e6e878ee3eeb7df2/e/777907623d5ddcf88ba4cbc9

To get these surfaces to look nice, you'll definately be needing some surface modeling. it depends on the quality you want to achieve.

Here's a basic setup you could use.
the seat basic surface is pretty ugly still, i just wanted to show options to tackle the problems you're asking about: arch, filleted edges and angled legs.

by manipulating sketch1 and the outer profile sketch, you can get a long way.

good luck!

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maybe the sketch inspector tool can aid in finding duplicate and overlapping lines in a future update…. 😉😉