1. Double room has a double bed while a twin room has two single beds.
2. You should opt for a double room with extra bed. You can mention that separately. Generally hotels provide crib but it's I guess for children upto 2 years.
3. Generally, 7am till 10 am
4. Yes, most hotels allow.
hope this is helpful.
Many Swiss hotels make a double bed by sliding two single "twin" beds next to each other and adjusting the sheet/ blanket/ duvet sizes.