1. If a good compost was used, it should hold your plants over for three weeks to a month. Using veg nutrients only serves to increase growth, but is not necessarily needed.
2. You have a lot of drainage in your soil as you mentioned. Both the sand and vermiculite increase drainage. You may have to water thoroughly more often or add composted peat moss or potting soil to the mix.
3. As of June 21, the days begin to get shorter and flowering commences. It's not the most favorable conditions for plants just starting their vegetative state, but it can and will be fine.
4. A three litre container sure is small. In that size, plants will struggle to reach the 4-5ft mark, yet alone produce well. Shoot for at least 2 gallons.