In nature, cannabis behaves as other annuals, which means that at a certain time of year, it goes into reproductive mode, flowers off, and then dies.
Indoors, you can force a flowered plant to stay alive, and with a bit of skill, revert it to veg mode. Having seeds on the plant makes this MUCH harder.
Yes, you can transplant in flower. No problem!
A cannabis plant takes from 7 weeks to a number of MONTHS to flower depending on strain. Start with an indica that is fast and bred for indoors.
Good seeds are fat, brown, and usually have dark stripes on them.
You can take seeds off the plant starting around 4 weeks after pollination, but for best results, give it 5, or until the calyxes are papery, tan, and dry.

The link in my sig might interest you.