You CAN leap into the sun, but you have to destroy it first. It leads inside the reversed pyramid in the Temple of Moonlight. If memory serves, you need the bombs to open this up.
Bisse: when I first played through that dungeon, I was stuck for hours until I noticed that there is a small opening above one of the walls, where you can throw a shuriken to hit a face thingy located on the opposite wall. I don't remember what it does exactly, though (I think it makes the trolley fall).
Marcus: you should be able to get the feather at this point, which is guarded by Algol, the blue giant in the surface. All you need to defeat him is the Serpent Staff. It is found in the Temple of Moonlight; you must hit some hard-to-see red switch several times from a platform that keeps going up and down. It's immediately below the reversed pyramid. The Book of the Dead, which grants protection against Anubis, is also found in this stage.
Regarding the long alcove in the Inferno Cavern, the tablet above shows what must be done. But there's a pedestal you must activate first to actually be able to walk through a portion of the wall near the bottom of that alcove. You will obviously need the Grapple Claw.
EDIT: oops, I think you were talking about the alcove in the lava pool. What I'm referring to is the precipice where there are flames coming out from the walls.
As for the boat in the Spring in the Sky area, have you tried hitting the wheel thingy in the bassin of the screen that precedes this one?
By the way, once/if you have the flares, you should go to the area east of the Temple of the Sun where the lights black out. Don't be like me and expect this place to eventually get illuminated through the activation of a pedestal or the death of a boss.