Will I lose the game if I dip below 0hp whilst there's a spell that'll heal me on the stack?
If I understand the situation you are describing correctly it is as follows:


A player at 2 life casts Sphinx's Revelation with X = 2. this is put on the stack.
His opponent responds by casting Warleader's Helix
The initial player in turn responds by casting Sphinx's Revelation with X=1
Both players pass priority until the stack is resolved or some other event occurs


This being the case, the following would happen:


Sphinx's revelation with X = 1 resolves, you gain 1 life to be on 3.
State based actions are checked (technically, twice immediately in succession as both players pass priority) and everyone is still alive
Warleader's helix resolves (because the players have initiated a shortcut of passing priority until the stack resolves), you take 4 damage to be on -1
State based actions are checked. You are dead. the game ends.
Nothing to see here, the game finished. Sphinx's revelation with x = 2 never happens.


State based actions occur whenever a player would get priority, not just when the stack resolves.