The idea is that "un peu xxx" does not mean anything toward the upper end of the scale--just the opposite. So when we say "a little warm", it suggests that it is in fact a bit too warm. That is not the meaning of "un peu chaleureux", rather it is "not very warm". Think of it as having "little warmth".

Yes, it would.

I wrote "The apartment was comfortable but a bit warm". DL corrected it as "The apartment was comfortable but little warm", omitting the article "a" and demanding "little" instead of "bit." Go figure.

You have the opposite meaning. The appearance of the article "un" actually reverses the meaning.

• mais un peu chaleureux = but a bit warm → actually too warm for you
• mais peu chaleureux = not warm at all → only a little warm, more cold than warm

This is a tricky grammar point, and it's not something you would hear everyday. Perhaps the choice of translation "only a little warm" is not precise enough.

I have reported my errors and have received no reply.

This is typical because we're a team of five volunteers and there are millions of French learners. We do a lot of error clearing in bulk. If your report is ever incorrect, you will not receive a reply. If your report is correct, you may get an email about it.

The construction peu chaleureux seems to be the same as peu profond, which means not very deep, or shallow.