I think that an invisible land-bridge would only help even the playing field for the AI. The AI doesn't defend coastal settlements in a meaningful way, making first strikes against multiple settlements on the French coast a near fatal blow to France, who rarely recovers enough to slow a player steamrolling them. The player on the other hand needs to place only a decent garrison in Edinburgh and Nottingham to ward off invasions. With AI vs AI situations, the corner of the map is more often stagnant and dull.
If the mod team is against the idea of an invisible land-bridge in the end, I am currently trying to learn how to make them myself anyways and will make a file available to anyone who wants it in the game.
As for merchants I am in favour of adjusting their abilities or leaving them as they are. I use them as a way to counteract the expense of military expeditions to far off lands. Distance from capital = less income from cities, but if you send merchants you can easily justify losing many soldiers for a mini empire on the other side of map.
Example: Player as Venice allies CS, conquer Egypt, send merchants to the most profitable areas around, then use this base of operations recruit soldiers, attack holy land cities for CS and sell to them for huge cash. Then open a multi-front war with ERE who usually conquers anatolia by now. Very fun but economically unsustainable without merchants. They make unique campaigns possible