Do you still have some he rounds left or have you already fired all of those?
I remember that the game tells you weird stuff like that when the unit don't have any he rounds left.
But you should be able to place the smoke target regardless. And as long as you are in the mortars limits (minimum/maximum distance) it should work properly too.
As a newbie, I have similar issues to use offboard artillry with 15 minutes + delivery delays.
Based on advices I picked here and there (and especially in this forum), if scenario length is less than 1 hour, as an attacker I start an artillery barrage right from the start (0 time delay) in harrass mode and maximum delivery time, all guns, on a likely enemy position. As game processes, I advance my spotters to a place where I can adjust the fire on other targets as required. It works pretty well. In this case, the delay command is pretty useful, when you are not sure to reach a good spotting point in time.
On the defense, I think you can pretty much do the same? You set a harrass mission as far away as possible, in direct view of your observer (as you are less prone to move it than on the attack), and you use delay if you estimate that the enemy won't reach this place in time?
Anyway, I understand that artillery on the defense is more an interdiction tool than a destruction tool: you fire on places you DON'T want your enemy to go, not necessarily on places he's likely to be.
I confirm that I found no way to add (or remove) guns to an artillery mission, nor to change the rate of fire.
It is worth to notice that Usually Hapless, an experimented player, mentions in one of his excellent YouTube AAR that, on the attack, the time to get FFE is so long that by the time he gets it he usually has found another way to solve his problem... 😉
