Hi, I'm at 61°N in Finland and I have a DIY biogas digester based on the SolarC³ITIES IBC design. It's part of my diploma in applied permaculture design. You can read about it:
https://www.beyondbuckthorns.com/content/dapd/introducing-jean-luc
-can biogas be piped into storage tanks for long term storage?
storage at the DIY scale is usually done in floating drums or storage bags. You can compress it - if you can get rid of the CO2
-can biogas be condensed similarly to liquid propane?
not on a DIY scale. At least not that I'm aware of.
-can biogas be used with existing propane fueled appliances, or do special appliances need to be purchased?
no. Methane needs a different air to fuel ratio - and biogas out of a DIY digester is without pressure. You can modify the stoves. There are videos on Youtube.
-will a small family with a small flock of chickens really produce enough to feed the digester and make enough gas to be of use for daily cooking or heating water in the winter?
depends. Temperature inside the digester should be at least at 20°C to get it going. 37°C is preferred. If you can get it up to that temperature it depends on the size of your digester. Let's say you have one cubic IBC, then you feed about 15kg of food scraps per day to get about 1m³ of gas.
-are there likely state or local laws preventing these sorts of setups?
If have no idea about regulations in the US...