The Carriage for Two is limited to one per account and you must have at least the Elyrian package to buy one.

In order to create a new character or take over an existing character, you must use a Spark of Life, there's no other way.

Souls have no lifetime, they last forever unless they're destroyed by some unlikely incident. However, a character has a set lifetime of a year with no deaths. Even with regular coup de gras, characters will still generally live for at least 3 real life months. So, you're not risking much. If you compare it to a regular MMORPG's subscription model, you get far more out of one Spark of Life.

I couldn't give you a definite answer on whether or not that is possible, but I wouldn't count on it. It's possible that you require a set amount of influence in order to purchase the Carriage for Two, in which case that wouldn't be viable.

Also, this "in-game currency" that you are referencing is known as Story Points. From DJ #17: Bolstering & Earn-to-Play:

Story Points (SP) are a post-launch currency awarded to players for being active and online. Put plainly, Story Points are awarded for anything that would lead to skill advancement or an achievement that you accomplish while logged on (not your OPC). This is true whether that be through completing tasks and important quests, by playing an active role in a growing economy, or by doing something noteworthy.

...

In Chronicles of Elyria, the Story Points you earn while playing can be traded in between lifetimes in order to buy Sparks of Life. This is what we call the “Earn-to-play” model of funding

While it will be possible to purchase a Spark of Life with enough Story Points, it's the type of thing that would take plenty of time and dedication to earn.