Personally I'd recommend having Elemental Blast as the modifier action, as you'll be using the key for Lightning Bolt more than Elemental Blast. Thus you can also just not hit the modifier for when you don't have Elemental Blast talented.
Code:
#showtooltip Elemental Blast
/cast [mod:shift] Elemental Blast; Lightning Bolt
If you still want your version, though, here it is:
Code:
#showtooltip Elemental Blast
/cast [mod:shift] Lightning Bolt; Elemental Blast
/cast Lightning Bolt
As for the space-saving macro, there are addons for it (though I don't remember the name of it), but natively I don't think macros support it. If the spell doesn't trigger the GCD however (check wowhead for the spell and see if the spell has a GCD of 0 seconds), you can list all of them as a /cast macro, and it'll fire on the first spell you're able to cast. Also, you could put a single spell that does trigger the GCD last. For example:
Code:
#showtooltip
/cast Astral Shift
/cast Stone Bulwark Totem
would work. It'll try casting Astral Shift, and if it cannot, it'll move on to the next one. Stone Bulwark Totem triggers a GCD, so the macro won't progress further than that, but that's fine as it's the last section of the macro.