Due to avas Iron man giving out defend, this question came up. If I have a line of characters, all with defend like so:

17,15,16,18,20

would the one all the way to the left get the 20? since he would be replacing his defense with a friendly's replacement?

Yes, Defend or any other defense sharing abilities (eg. JSA, Defenders, etc) can be chained from one to another so if all of those had Defend, Defenders, et al, would be able to share the 20 defense value.
Same with Batman Enemy sharing attack value.