I think it's so unique that you give the player 1 HP that it'd be a huge shame to change that. Or at least to change it early.
As for shops, I think that if you're gonna have a town or shops or such it'd be much cooler if instead of currency used to BUY weapons/health, you gave the player a weapon, then included multiple NPCs who would gladly trade an item of theirs for that weapon or one of the other NPCs' items--including one who'll trade you a 9HP capsule for the gun, one with a different gun, maybe one who'll trade you the air bubble, and so on. Have it be revisitable and reversable. Then, in the same fashion as the island part of Zelda, Oracle of Ages, have clever use of one of these items be able to get you to, say, a gem that you can trade for an extra item so you can now carry two at once, and so on until you have all of them. Maybe make the booster your initial bargaining chip rather than a gun. Mostly, though, I think it'd be much interesting if you made the player trade an already useful item for another one than an arbitrary unit of currency, even if said item was handed to them just before the town part and designed just for that purpose.
As for the spikes, if practical teleporters are better. Rather than being a complete slap on the wrist though, make the "spikes" teleport you back two jumps--and make you come out of those teleports with the "fallling upside-down" sprite from the normal ending, land in pile on the floor, and get up to try again. Make em still LOOK like spikes though.