Artillery and Small Team

The rule applies to units that ‘consist of either one or two infantry models’ – artillery units by definition consist of the gun model itself and a number of crew models, so it can never consist of one or two infantry models (the FAQ confirms that the intent of the term ‘consists’ as excluding anything else).
“If I shoot at a unit of artillery that has only one or two crew models left, does the ‘small team’ to-hit modifier apply or not?
The small team rule applies to units made of one or two infantry models (implying ‘and nothing else’, which should have been stated clearly). So, so as long as there is also a gun model, that unit is not made just of one or two infantry models, but also includes a non-infantry model, so the small team bonus does NOT apply”

No.  Mortars and Machine guns are infantry, and more specifically for this rule, infantry support team weapons.  The weapons are not distinct models within the unit, even if you model them on their own bases.  See p35, Units, Types of Units for the definitions of infantry and artillery units – only artillery defines the gun as a model within the unit.

They are man-portable weapons by Bolt Actions definitions (though some heavy mortars really blur the line).  They are effectively the firers personal weapon, so when you lose him you also lose the weapon.