Welcome to the forum corcor.
Some answers for you:
1. Yes. Not looking at the map but land units can cross the Turkish straits, as Turkey is a single territory. If either side attacks any strict neutral all other strict neutrals become pro the other side and so can be occupied in non-combat, exactly as you say.
2. Yes. For a minor factory must have 2 ipcs and can build up to 3 units. Major factories can be built on 3+ ipc territories originally owned by a Power and produce up to 10 units.
3. Yes. Generally, any land or sea unit(s) can retreat from a battle whether it has used its full movement value to get there or not. I say generally due to the exception that land units conducting an amphibious assault cannot retreat. As you point out retreat is therefore a useful tool at times for gaining movement advantage or consolidating forces.
Cheers
PP