Given your edit, you technically have 2 separate questions to answer, with a special condition. For this reason, I am giving a three part answer.
By RAW, a Returning Weapon does not move other objects.
Also by RAW, the Returning Weapon only returns after making an attack, preventing the listed scenario about the cart.
Given that the shield appears to be sentient, the normal limits of Returning Weapon may not apply.
Returning Weapon states
prerequisites: A simple or martial weapon with the thrown property
This magic weapon grants a +1 bonus to attack and damage rolls made
with it, and it returns to the wielder's hand immediately after
it is used to make a ranged attack.
So, returning weapon, by UA RAW, can only be applied to a weapon with the Thrown property. A shield is classified as being armor, not a weapon, so this infusion cannot be applied to it RAW.
Further, Returning weapon states that 'it' returns to the wielder's hand, and makes no mention of it bringing anything along. So it explicitly mentions the weapon itself only.
Finally, the infusion only applies after making an attack with the weapon, which means that the user cannot attach it to an object, then summon it to them, as that is not a form of attack.
Since you state in your edit that the shield may be sentient, it may not need to have the Returning Weapon infusion to be mobile, it may be able to move on its own. In this case, you need to work with your DM to determine its limits of capability, such as carrying capacity, or push/drag/lift capacity, speed, and what conditions negate its abilities.
If it still requires Returning Weapon to be mobile, it likely has no capabilities beyond sentience and returning, but, again, you need to discuss this with your DM.