Ahh, I see it now. Missed the page number for that section, so I was thinking it was tied in with the advanced rules. Thanks for straightening that out for me. NOW the 42" makes sense.
Let me try to add references for all this, just to double-check this is all accurate. So, you can still do direct or indirect. Your direct to-hit mods are, with no advanced rules:
Artillery Unit's Skill, p.5 errata, references p.34-44 rulebook for standard attacks.
Attacker Range Mod, Long Range [+4], p.5 errata, to-hit number section
Attacker Movement Mod [+variable, can be -1 for standstill], p.5 errata, references p.34-44 rulebook for standard attacks.
NO TMM mods (including immobile) for POI targets (and why would you ever choose to shoot at a fast unit when you have this option?), p.5 errata, to-hit number section.
NO spotter bonuses. nothing in the rules says you get them for direct fire, so you don't.
Terrain Mods between attacker and target [+variable], p.5 errata, to-hit number section does NOT say these are ignored, so standard attack rules in effect.
For indirect fire it is:
Artillery Unit's Skill, (same as above)
Attacker Range Mod, Long Range [+4], (same as above)
Attacker Movement Mod [+variable, can be -1 for standstill], (same as above)
NO TMM mods (including immobile) for POI targets, (same as above)
Indirect Fire penalty, [+1], rulebook p.37 Misc. Modifiers table.
Spotter Range-to-Target Mod [+variable], p.3 errata, New Indirect Fire text
Spotter Movement Mod [+variable, can be -1 for standstill], p.3 errata, New Indirect Fire text
Terrain Mods between Spotter and Target [+variable], p.3 errata, New Indirect Fire text
Spotter has a Probe special, not covered by ECM, [-1], p.5 errata, Indirect Fire section.
- OR -
Spotter has TAG special and artillery is Arrow IV, not covered by ECM, [-2], p.5 errata, Indirect Fire section.
Looks good. I was able to map pretty much all of what you were saying to the published rules. Congratulations, you do, in fact, know what you are talking about! However, there are a couple things about this that stand out to me. First, the PRB, LPRB, and TAG specials are otherwise handled in the Advanced rules section, but they are included in the standard rules for artillery. Maybe letting you have access to those bonuses without asking you to use the advanced arty rules is a mistake? I know folks can pick and choose advanced rules, but if you pick the standard 'easy' artillery and then take advanced specials/rules that help counteract the difficulty of landing shots in the 'hard' rules...well, you are kinda choosing to be unbalanced. It wont fix everything, but maybe Probe/Tag don't belong in standard rules. I'm sure there was already a huge fight about that somewhere that I missed, but I'll throw it out there anyway.
Second, stacking standstill mods is probably a bad idea. Honestly, I'm not sure we should include standstill mods at all for artillery attacks, but at the very least we probably don't need to include the movement and range mods from BOTH the spotter and attacker since it appears that can be used to get a -2 to-hit by having both stand still. I vote take the range mod from the attacker and movement mod from the spotter, but I'm open to other ideas.
Third, I don't like the indirect fire at units in line-of-sight. Yeah, yeah, I know, you are shooting at a POI that happens to be a millimeter away from their feet. Still seems cheesy and overpowered to me. I think I'd prefer if direct fire attacks didn't ignore TMM, regardless of target type, or you were not allowed to choose a POI which would include a unit that you have direct LOS to. Otherwise, there is virtually no reason to use direct fire on a unit, even if you can see it, and that seems wrong.
Finally, how in the world did Watchdog (WAT) end up in standard rules! Maybe instead of having the WAT special, it should have just granted ECM and LBRB to the units that had it. That or maybe we can add it to the list of 'probe' specials in the errata. Of course, it looks like there are only 15 units on the MUL that even have this special, so it might not matter!
EDIT: One more thought. What if we don't let Forward Observer stack with PBR/TAG, or say it replaces the mod for those specials? Combine that with not letting both spotter and attacker add -1 for standstill, and it pulls you back from the brink of auto-hit silliness.