With this current setup there really isn't very much room for a good alternative jump button. Spacebar is fine, but I think there needs to be much finer control over the character for this to work well. After playing the entirety of Super Meat Boy with keyboard controls, I can say that they do work but there needs to be an incredibly solid base which I don't think Last Legacy has.
I think at this point you're kinda there and I am willing to forgive non-perfect styles for game that is both from an inexperienced developer as well as done in your free time. As for feedback, well...
-This may not be a priority for a free flash game, but I think your general gameplay needs fine tuning. The feel of your character needs to be responsive and follow expected physics. There doesn't seem to be any acceleration, and therefore deceleration, which makes predicting movement much harder as it's all so sudden. I would extend fine tuning to combat as well as I feel that, with at least swords, it's a little clunky. It may not be the intended focus but, with the way the levels have been designed, combat is an integral part of the game. As of right now, I don't think it's particularly fun to attack.
-I wouldn't say any of these levels were particularly amazing, and I think that is mostly to do with flow. Too sudden a change of direction (in terms of design) ie slow retread after being complete freedom in speed. I would also extend some of the issues other people have said - too short. Some ideas I think aren't explored far enough while others you spend too long doing (block/switch puzzles)
-Clicking for switches, doors and chests needs to be replaced with a button(s). Clicking is just another step that gets in the way of the player and continuing to play.
-Add pause to Esc. With one hand on the mouse, the most logical position for the pause button needs to be somewhere you can reach with your other hand, which for most people is Esc.
-Add E for your inventory. The same reasons as above. It's a lot closer and it's (currently) not used for anything.
-Maybe get another sprite for the character when it faces a different direction. As I think I've mentioned before, it's just been flipped so the character just switches hands depending on the direction it's looking.
-With some people crying for a non-mouse setup, I felt Dustforce had a good enough control scheme that it could potentially be mimicked for this game.
Jump - Z
Attack1 - X
Attack2 - C
Movement - Arrow Keys
Aiming with projectile weapons becomes a lot more restricted for the player but with a mix of arrow keys (ie up + right for that direction projectile) at least is could be an alternative scheme.
Beyond all this though, I enjoyed my time with this preview. The introduction to RPG mechanics is nice, as well as a use for all those trinkets. We have much more of an incentive to explore the level now.
At least, I'd call it a success. It's very ambitious for a flash game and to get to this stage is impressive. It's some good work. :D