In more detail:
1) & 2) are really country-dependent. As said above, it depends on the class of ATGM you are talking about, US being the exception with overwhelming quantities of short-range weapons. As far as I have seen in the different countries I have researched, medium-range ATGMs are generally battalion-level assets, even though the actual units are often cross-attached to lower levels (say one 6-ATGM platoon at btn level give one 2-ATGM section to each of its 3 infantry cos).
That's what you describe as your Eastern model, but it should work for most European countries and those working on the same model. Let's say:
-1 to 3 medium-range ATGMs per infantry
company. When not available (e.g. US pre-Javelin), substitute with long-range ATGM.
-1 to 3 short-range ATGMs per infantry
platoon when available. In most cases these will replace unguided AT rocket launchers (RPG-7, Carl-Gustav, Panzerfaust). Depending on the country, these heavy AT weapons were assigned one per platoon or one per section.
3)As you said, most Western TOEs had huge battalion-level anti-armor allotments (at least until the 90s). In many cases, these would not necessarily be manned to full strength, so I'd suggest that it would be more about AT assets being stripped of or converted against low-armor enemies. Typically, a US airborne battalion would have about 12 TOW-armed Hummers in its tables, but the TOW launchers could be replaced by heavy MGs or AGLs if not needed, so you would convert from a mobile ATGM-jeep squadron to a mobile infantry support/recon force.
In Eastern countries, you may have more AT reinforcement, as the basic TOE included less low-level ATGMs, but the standard Soviet division had an integral anti-tank regiment (MTLBs with AT-6 SP). Also bear in mind that most Eastern countries have gone on using non-ATGM anti tank weapons (AT guns, recoilless rifles) much longer, so this may shift the balance and suggest there were unduly low level of AT weapons if you count only ATGMs. For instance, the standard AT-3 platoon had 3 launchers, supported by 2 SPG-9 RCLs and 3 RPGs for short ranges.
4)Basically, tank units don't get many ATGMs since they are their own tank killers. That's particularly true about modern MBTs, which are much more antitank-oriented than previous models, and much more accurate. Playing 60s-70s forces, you can add one SPATGM platoon per tank battalion if you have high-end forces.
Also mechanized infantry gets less organic ATGMs. Particularly Soviet-style BMP units get zero infantry ATGMs while BTR infantry gets one short-range platoon per company (when available) and at least one medium-long-range platoon per battalion. The basis being that the BMPs carried their own AT weapons (see mixed AT platoon above).
Of course, mechanized or armored infantry has more chances of getting SP ATGMs.
5)In most cases, vehicular were used to replace foot ATGM one on one in mechanized units. SPATGMs are generally higher-level assets when medium-range foot ATGMs are available, because most armored ATGM carriers use long-range missiles. Jeep-mounted ATGMs are either substitute for armored carriers in light units or field mounts of foot-borne weapons, as said before.
6)No idea about the actual mix, but keep in mind that the USMC is not exactly an armor-heavy force. I guess this is reflect in the game picklists, so if you run a battle with a large IA-purchased USMC force, chances are that the computer will purchase not a lot of tanks and a fixed amount of ATGMs based on the force size. If you want to have a less balanced, more armored force, better pick the units yourself.
7)See above, most Middle East countries have a mix of British and Soviet structures. Without much actual info, I repeat the baseline that ATGMs are more about supporting infantry than supporting tanks. After all, tanks once used to be more about supporting infantry.
Some actual orbat links:
orbat.com public library
Some wargame-based TOEs
and some more
and then some
Also make sure to check out the US battalion TOEs and the Field Manuals posted not long ago here.
Hope this helps.