I am not a fan of the recent changes in market and diplomacy that reduce the player interactions. However, here is a proposal for design improvement to get back some of what was lost while adding to the game:
Political Research Tree (like Infantry/Armor/Air/Naval/Special)

These can be introduced over days of availability and at different costs to be determined.

1. Trade Pact
Allows you to form a trade pact with any other country that has a trade pact
This allows you to have direct trade without going through the market or within your coalition.

2. Favorite Nation Trade Pact
Allows you to trade direct with another nation without having to pay the trade commission on your sales or purchases

3. Grand Coalition
Allows you to add +1 member to any coalition that you are the leader of

4. Alliance
Allows you to make an Alliance (Not a shared map or intelligence) with another country that also declares you as an ally. Allows you to trade units directly daily up to a minimum of 2 units to a maximum of 5% going either way: to or from the ally.

5. Grand Alliance
Identical to Alliance but up to 4 or 10% of the Armies.

6. Supreme Headquarters Allied Expeditionary Force (SHAFE)
Allows the nation which has the SHAFE to build a Headquarters to receive authority from the other nation to do the following:
Order units in the adjacent provinces or range, of all allies up to a limit of a certain number of units (4-6) a day

Multi accounts have much more serious offense than just trading a few units.
To shut down an option because of that part of the problem is short sighted and reduces the attractiveness of the design.

By using the method above it allows for things to happen in a gradual and reasonable style.