Q1 - really no need to change the cassette with every new chain - except - if you've ridden the chain until it's stretched and worn the most used cogs beyond the point where they'll work properly with the new chain.
Best is to measure the chain, if it's between .75% and 1% stretched, then definitely get a new chain. Doing this assures your cassette will have max life. I have cassettes with 15K+ miles which are still fine...
Once the chain gets stretched beyond the 1%, the likely hood is that the most used cogs will be worn beyond the point where they'll work with the new chain - you'll get chain skip on those cogs.
Buy the new chain - try it with the casette first and see if it skips on any cog, under a good heavy pedaling load. Chain skips, consider a new cassette. Chin doesn;t skip, you're good to go...

Q-2
For me cog range is more important than 'model'. If you like that 12-27 then get another 12-27, they are easily available. If the LBS doesn;t have one, they should get it right quick.
I find an 11 fairly useless for most of my riding, to have it one gives up a much more important, more often used cog in the middle of the range. A 12 does all of the 11 and doesn't need the sacrifice of another important gear. 27, 28 it's all pretty much the same.
Sounds like your LBS needs to hard sell less and accommodate your needs more...

EDIT: a chain measuring tool is $12 or less (down to $6-7) then you can decide when you need a new chain. Simple measurement you can do in 30 sec. every now (if you ride frequently) and then (if you don't ride that often).