@Gnarly
On-map mortar smoke barrages are possible (I just tested it), but not all mortars have smoke shells in the first place and if they do it's usually not in large or even meaningful numbers. To use smoke/WP shells, you either use a FO like for an off-map mission (minimum and maximum range restrictions might be felt more than with heavy off-map pieces) or you use the Target Smoke command (provided the mortar team has LOS on the target).
For example, I tested a January 45' quick battle as the U.S. Army (forgot about the wind, I assume it was "normal/medium").
60mm light mortar team - no smoke/WP shells
81mm medium mortar team - 2 WP shells, both Target Smoke and Smoke fire mission worked although I won't try to convince you that a 2-shell barrage is very effective
As for your difficulties with the indirect fire mission system, well, I share your struggle. It must be hard to go from modern-everyone-has-radios-and-artillery-is-a-tick-of-the-clock-away to WW2 standards. Unfortunately, there are no easy solutions, it's all about using what you have at your disposal (be it time, a good position, FO's, etc...). Don't be shy to buy/borrow some jeeps to either transport your tubes or keep C2 (a jeep will take in a mortar team, but not the two ammo-bearers). I find that one of the biggest barriers to effective use of artillery is psychological : fearing that a fire mission will land too late and either be ineffective or worse, kill your own men who have advanced. For this, there are two solutions.
A : you can rather easily adjust or outright cancel an ongoing fire mission (just select the unit calling the mission).
B : just do it! When you plan an attack, you have a tendency to assume that things will go according to the original plan. That is sadly rarely the case, but that means that you will find that the 8-13 minutes or so required for the bombardment to start in earnest are much less significant in terms of your attack's timetable (chances are, you'll still be trying to achieve fire superiority over that hedgegrow, among other outcomes). Worst case scenario, just cancel the mission, I'm pretty sure this is done almost instantly and you should be clear to proceed after 1-2 minutes (I assume you won't see projectile flight-times much above this).