Renaming the field to summary would be an easy change. I'm not sure yet how easy moving the field would be.
You mention using the summary content for link previews. Note that summaries do not support BB-Code, so the previews would be somewhat dumbed down from what they are now; this change should be an option. I'm unclear what would be the desired behavior for section links (which preview the section text currently), since that would become inconsistent with the behavior of regular link previews, and sections don't have separate summaries to pull from.
Summary is already present on wiki content variables (varies depending on the template) via the "summary" key. So for vw_node_shell, which is where you would probably use it in XF2, it would be accessed like {$node.summary}. I imagine you would want to insert it either above or below the line for {$node.below} in the p-description block. IIRC, feed descriptions already appear here, so that might cause a double-description on feeds if you simply edited this template. The page description was not placed here by default since the values carried over from VW 4.0.x tended to mostly be duplicates of the page content.
Thanks for pointing out that wiki pages with empty summary fields reverting to the site description is not a desirable behavior. So far, I only see this happening on my vBulletin boards; I'll see if there's an easy way to fix.
I'm not sure I would lower the maximum as a hard value. Historically the recommended length of this field by search engines and other experts has been all over the place from over 300 characters to under 140 characters. A few years back 192 was the maximum some engines would show (but they might index the full value). Google tends to revise their guidelines every few years. Any change we would make would only apply to new edits -- much like if you were to change the Maximum characters for wiki content to 100, it would not let you save future edits until you came under the new maximum, but it would not destroy your page history.
For both summary and I realize even content where the maximum is different between versions, I am not clear what should happen if you tried to roll back... nor am I sure what the desired behavior would be (error: can't rollback to that version because it exceeds the maximum allowed characters)? Not sure it rises to a Permissions Escalation, because it's not like you're adding new content over the maximum, and really only moderators should be able to roll back, and one might expect moderators to be able to bypass certain limits in their official capacity (but should moderators be above the law?)