Great questions. Reciprocity absolutely factors in, depending on the length of the exposure and the film being used. For example, Fuji Acros 100 is famous for having very little reciprocity failure - none up to 2 minutes of exposure. On the other hand, reciprocity failure starts kicking in on Kodak Ektar after just a couple seconds. Reciprocity is of course needed to maintain optimal exposure, but many films have a great deal of latitude and a useable image can be recovered even if it's off by 3 stops in some cases. As to what reciprocity failure graph is right is up for debate though - the manufacturer's spec is a good starting point, but many photographers get to a point where they dial in their own graph based on their own testing - it's what works for them and their process. Personally, I'm happy with the manufacturer's spec and would rather be out shooting than in the lab testing for the perfect exposure.
As for expired emulsions, again, the spec is a good starting point, but it's a little bit of a gamble. A lot of times you can google to get some level of predictability. Most color films go through a color shift, and maybe lose 1/3 of a stop depending on age. But google will usually turn up some results of people talking through what they've found for a particular film. If all else fails, just try it following the spec and see what you get back, then shoot the rest of the expired batch with the learnings you got.
Keeping a semi-decent shooting record will help you improve, especially with the expired film. Thanks again for the questions - I'll note this as a possible article to expound on in the future.
Cheers!