Hi Hash,
I'm a newbie as well so everything that I am writing below is based on what I've read over the past couple of months. I figured I would reply with what I've learned since no one else has so far.

1.First signs of sex: I assume you are growing indoors and recently switched to a light schedule of 12/12. If this is the case you should be able to determine the sex of the plants within 1-2 weeks. I'm currently 1 week into flowering and have been able to determine the sex of 2 of my 4 plants.

Harvesting: You normally want to harvest 6-10 weeks after switching to 12/12. From my understanding you want to harvest when 50% of the trichomes are amber colored. I've never gotten this far in a grow so this is based solely on what I've read.

2. Post-Harvest plant: I think the plants can stay alive after the harvest and can be switched back to vegetation. I think the buds that you get after from this plant on the second harvest will be lower in THC. Have you considered cloning your plant?

3. Rasberry weed: I know nothing about pollinating marijuana with rasberry. My guess is that either the plants can't cross-pollinate or if they can the buds will be lower in THC since only 1/2 of the genes are from a marijuana plant. This is based on no actual knowledge so please don't take it as accurate.