Welcome! I'll give answering some your questions a go--
1. Burgos or Astorga as starting point. I hope to finish and arrive in SDC in 12 days. I have been planning Astorga until this afternoon
I'd walk from Leon or Astorga if I was planning 12 days.
2. Recommended place to stay in Astorga- albergue, pensionne or even hotel - if I do start from here.
We stayed here:
http://www.caminodesantiagoastorga.com/index.php?ididioma=3&modulo=2
It is a great albergue, and has beautiful views.
3. I have 3 more weeks of “family travel” after my Camino so I plan to send my bigger / heavier pack from Burgos to SDC with Correos. Will it be easy / quick to claim it at SDC? Any other company that can do this?
You can send it to Ivan.
4. I also want to do the Finisterre and Muxia caminos - is this possible in 5 days if I average 20 -25 kms. per day.
You can at least do one way and then take the bus back.
5. Recommended app with detailed route and information of CF. Something that will work offline would be great. My biggest fear for this Camino is getting lost / off track since I will be walking on my own.
You won't get lost. You probably won't be alone the entire trip that time of year. That is, there will be other walkers ahead and behind you. There are a thousand arrows. You don't need an app. Turn off your phone. Connect with people if you can't find an arrow. (Me pierden. ¿Dónde es el
camino de Santiago de Compostela? Gracias.)
6. Anybody used “portable WiFi” recently? Which company did you obtain it from? Is this better than getting a local SIM?
Nope. I got a local SIM in Leon when I arrived.
7. How is the weather mid-May to early June along the CF?
Beautiful. Count on rain.
8. Any other recommendation for my 1st day of walk? I have requested my credential from americanpilgrims.com (thanks guys) so no need to worry about that.
Take it easy the first few days! Go to the Pilgrims' blessings and mass even if you're not Catholic. Light a candle at the churches you pass.