Yes.

If no selection restrictions appear in the variant army list, there are no restrictions on that unit type in the variant list. So, Knights of the Realm are not a 1+ choice. Multiple units of White Wolves ARE allowed in a Middenheim army list, and Von Carstein Vampire Lords are NOT 0-1 (it's a bad idea to fight in Sylvania). Any exceptions to this are noted in the relevant army sections.

Players should refer to the Army book unit entries for everything but selection restrictions (as noted above). In this case that means that if they are allowed more than one unit as a single choice in the standard Army book list, then they are allowed more than one unit for a single choice in the variant list as well. Thus, for instance, two Bolt Throwers in a Sea Patrol list may be chosen as a single Rare choice; two Spear Chukkas are a single Special choice in Grimgor's 'Ard Boyz: and two Chaos Spawn are a single Rare choice in Archaon's Horde.

Yes, unless stated otherwise, the relevant special rules in Army books are always used. Thus, Dwarf Slayers in a Slayer army ARE Relentless, Unbreakable, etc., even though they are not noted as such in the Slayer army list.

Only where specifically stated (e.g. the Dark Elf magic items, the Hellcannon, and Malakai Makaisson's Goblin-Hewer).