I didn't get to follow through with my multicolor stain idea because Tandy and Michaels don't have regular tan antiquing gel. I ended up going with varying coats of saddle tan and the effect is almost what I was shooting for.
I got the seat mounted onto the seat pan and do not recommend the Tandy rivet setter. It's a pain in you know what when working with curved surfaces like a seat pan. I bent 5 of the rivets which is a drag because it scuff the leather a bit. Next time I will lace it.
Here is a pic of the finished product, minus the satin sheen. You can see some of the obvious mistakes but overall I am pleased for a first effort (the dark spots are shadows).
I am already planning the 2nd version that should be much improved because I leaned a lot on this project.