These are not the scenarios that raise concerns. What raises concerns is for the possibility to send 40 000 CS or more, on a realm that doesn't have all of its forces constantly around the capital (either due to being larger, or to be active abroad).
40 K enemy CS in your realm is indeed something to concern your self off, may it be sea travel or land travel.
Sea travel has a little more of the element of surprise in terms of when the enemy is coming.
But dont forget that sea travel is very very expensive, and still quite a risk even for the attacker.
Besides what an attacker does the defender can do as well, you mention for example the defender to be abroad, that means that as long as the enemy torches your land, you can torch his?
Let us just form opinions like this a bit later on, when we learn more.
Right now we can only make assumptions, and i found yours to be rather extreme right now.
