1. Mixed-signal design is very specific, and it require some skill to avoide failure. High accuracy analog part require a lot of efforts in integration with noisy digital core,- it is a separate song!
2. There are a lot of IPs on the market, practically everything can be bought. It is a question of money, project budget, which is typicall limited. An experienced designer from company, having good relation with a chip manufacturers, can reduce IPs cost by proper choicing the manufacturer.
3. Strongly depends from technology node and relation between analog and digital parts,- analog isn't scaling such good as digital. In 0.18um node such chip can be about 10sq.mm.
4. For mixed signal with significant custom design portion Cadence is preferable. Verification is better from Mentor. Digital design with external analog IP usage - Synopsys.
5. Back end mainly. FPGA is based on ready blocks with a limited flexibility. In ASIC design you have more freedom and more risk in design.
6. Necessary to define preferable specialization direction. General descriptions are practically usefullness.
7. A lot of else, but not in brief description