I'm wondering if it is possible to just launch a QuickFields session to process documents that are already in the LF Repository and print them with the redactions without having to resave a copy in the repo. 

No, you can't print out documents using Quick Fields, you'd need to store the modified pages back to Laserfiche and print from there. 

We are also looking at auto redacting a SSN and DOB but want to only redact the first MM/DD and leave the YYYY only in certain regions of a page. There's no convention with the number of pages, so a report can be anywhere between 2-20 pages and we would need to redact the social and DOB in give regions. 

You can use Fixed Annotation if the data is in a consistent region on a page, or you can use Auto-Annotation if you're able to use pattern matching to extract the text you want to extract. What will work best for each situation depends a lot on what else is on the pages. A few questions:

You said the report can be a variable number of pages, but are the pages you need to redact from in any sort of consistent format?

Is there other text on the page that looks like MM/DD that you'll need to make sure you don't accidentally redact too?

For the documents as they exist in Laserfiche now, do you already have the corresponding SSN and DOB in metadata fields for each document?