Well, you're in for a ride. Solid stain's application is really no different than paint's. For woods like Cedar and Redwood that bleed a lot of tannin, you'll need 2 coats of primer after sanding down to your desired look.
Meaning, if the Cedar has heavy grain like the first picture, then you only want to lightly sand. If you want smooth, then you'll sand a ...
White vinegar is considered a simple solution to grease stains. Some vinegar, dishwater detergent, water, and glycerine together can help remove the stain. Start with some water by sponging the stain, then apply the other liquids via an absorbent pad.
Dab at the stain and allow the pad to absorb the grease over time, swapping out the pad when it gets too ...
"...stick with the same spar urethane...?"
"Should I expect a fresh coat of urethane to give the stain a more even finish"?
KINDA The spar varnish will slightly darken everything, making the wood look like it would look if wet. Take a damp sponge to it to see if you like the look.
"do I just need to re-stain first?"
"How many coats ...