"1) Is there substantial added value in going with the $60 Shimano Sedona versus the $20 Shimano AXULSA? Will the more expensive rod cast further assume the same 4lb mono on both and the same rod?"
That is a definite maybe. If the spool is smaller diameter on the AX-UL, then it will not cast as far. Larger spools will cast a bit further. But, like any good equipment, the difference between a 3 bearing AX0UL and a 5 bearing Sedona is mostly in comfort. Using 4 pound test line, a big fish could outrun the line you have. Simply open the bail and drop the tip of the rod if this looks like it will happen. Usually the fish will stop running. A 5 bearing reel will generally be smoother and more consistent in reeling action. Less stress on the hook, less stress on the drag, less stress on the rod and, oh yeah, less stress on the fisherman. Often machined spools are closer to true than an injected (plastic) spool. They do not wobble. Gears are a bit stronger and hold up better. I usually fish 2pound test line. If I catch something larger, I know I can beach him in most cases. My drag is usually set to just hold steady on a lure retrieval. I use a pumping action to reel in a big fish and snub the spool with a finger on a longer run. I have landed fish in excess of 10 times the line strength(21 pound salmon) on little size 16 or 14 hooks. And several hundred steelhead with their tail walking antics on light line on similar. My brother calls it "cobweb" and mini-lures. It works for hooking fish. I believe Lee Wulf landed a North Atlantic Salmon on a size 18 fly and 2 pound tippet.
2) My western rod is a Reddington Classic Trout 9ft 5wt. My now-broken spinning reel actually fits onto the rod and I successfully fished fly/bubble with the spinning reel on the fly rod for a few casts. Any reason this is bad for a fly rod? If so, I'll likely pickup a new 2-pc UL 5'6" spinning rod (either bass pro or shimano). But if it isn't likely to damage the fly rod, I might just use the new spinning reel on the fly rod for now. With the bubble on, it was actually pretty easy to do a sort of weak pendulum cast and throw the bubble pretty far. Plus my fly rod and tenkara rod fit nicely into my rod holder.
Well, again, you get a maybe. The tip may not be (often isn't) ceramic. Monofiliment can cut into the tip. It was common to carry a spare when you started breaking fish of at the rod tip 35-40 years ago. You heated the tip with a match and pulled it off. Then heated the new tip and pressed it on (with a bandana, of course.) Anyway, Fly rod tips are not really designed for mono. Replace the tip with ceramic. By shortening the mount and removing any braces, it will work pretty well for either. Also the larger eyes on a spinning rod will gradulay reduce line flutter during a cast. On a fly rod, not so much. Leave at least 24"-36" before starting to thread the guides, always skip the first one, maybe two or three, depending on how the guides are arranged, the size of your spool and weight of your line/lure. I use a 9'6" noodle rod, that is a rebuilt fly rod for steelhead fishing in winter. I use spring steel to make my own guides on various rods, but this is a custom technique. They are cheap enough to buy. often the snake guides are better in winter because of icing, they clean easier. It doesn't matter much inbetween, 95% of the flutter is picked up on the first guide while casting.