Politeness throughout the sentence

Hey, I am very confused about using polite forms in front of sentence connecting words.

My book (Marugoto A2.-2) introduces e.g. the ので grammar with the pattern:
sentence (plain) ので、sentence (polite / plain)。
Which would indicate that, well the first part is always in plain form.

Maybe I am hillucinating, but I feel like I have heard certain sentence connectors with both polite and plain form in the first sentence.

For example:
1. 時間がないが、行ってみます。
2. 時間があるませんが、いってみます。

or

1. おそいから、帰ります。
2. おそいですから、かえります。

Is both okay?
If yes is 1. standard politeness and 2. very polite?

Thanks 🙂