1. As Infacta states.
2. Turning down the boiler stat will cause the boiler to "short cycle". It will reach the low temp quite quickly and stat out (turn off) and when temp within boiler jacket cools below set temp, the boiler will re-start. Short cylcing uses a lot of oil. Boilers are designed to chase heat rather than the above. You are better to set the boiler to come on say once an hour for say 15 mins. Otherwise fit a frost stat in conjunction with a pipe stat. Turning down the room stat inside is as Infacta states.
3. As above.
4. The numbers on a boiler stat are not a percentage but a temperature. The most efficient setting is between 70C and 75C. For non-condensing boilers, you want to balance the system to give you a 20C temperature difference between the boiler flow and the boiler return. For condensing boilers, you want a temp difference of 10C. You can also do this be fitting a thermostatic valve between the flow and return set to 50C with the flow temp set to 70C. Internal design room temperatures are generally 21C and 22C for bathrooms. Higher for elderly people.
5. Again with a frost stat fitted along with a pipe stat, you do not have to worry about temp drops as the frost stat will have a permanent live to it, independent from your timeclock. The pipestat should be set to 30C to turn off the boiler when the water temp reaches 30C. The frost stat will kick in at 4C. It's not an expensive job either.