Really great style here.
Instant appeal
In response to ur questions...
just an idea, have u considered the lava is flowing into the temple rather than just sitting there?
It's both logical as to why the place hasn't melted as well as an opportunity to make some exciting animation that would add to the sense of danger and urgency.
for example the lava could be splashing against the platforms.
You could even make it a gameplay element that there are sometimes larger waves of lava u must get over.
In terms of lighting..it's an option to go darker further down the screen so the underlight from lava stands out.
Some more structural details and things like statues etched into the stone would help reinforce the effect.
No harm in considering a third light source for ambient back lighting either.