So I was going over から, and I noticed how all the sentences had like 病気だから, or りんごがない in the middle of the sentences, and then they had keigo/formal at the end with ます and です. So that got me thinking, is keigo/formal just never used in the middle of setences, and it's always the "short" version that is used?
Your use of the word “keigo” is a bit misleading. When people hear it, they usually don’t think of です and ます although they are technically part of “keigo”. I will call them polite forms as opposed to plain forms, which you called “short” versions.
Some conjunctives do require a plain form but から is not one of them. You can use a polite form before it.