I'm a newbie so I can't really answer most of your questions, but I came across this article the other day with a helpful seed starting chart.

Here is a link that might be useful: Organic Gardening Seed Starting Chart

[is hardening off time typically included in the instructions, "Start indoors 6-8 weeks before last spring frost"]
Yes it is.

[Is there really a big difference in 2 weeks succession between plants of the same variety vs. starting them at the same time?]
Yes, if you want a continuous crop of a specific item.

For example, I like fresh spinach and lettuce, if I plant seeds every week or two for 4-6 weeks I will have it available for a longer period of time.

It is really whatever your preference is.

I found a really nice form with all the "normal" plants for gardening listed it then goes further to show the zones and has a fill it in section for each one to answer when to start and when to harden off and transplant. It is from Mother Earth News and I am going to give it a try this year. I am trying to plant extra to take to my local farmers market and to get that much I need to start alot more seeds! I also want as many of the plants to be heirlooms so I have had my nose in the Baker's seed book everyday for the last few weeks!!! So many choices, it is like being at the candy counter as a kid. LOL The link is below hope it helps someone else.

Happy Gardening

Here is a link that might be useful: Mother Earth News Seed Starting Guide