Who is best to train with?
1. Somebody much better that yourself. A great opportunity to learn by feel. Unfortunately, rarer and rarer as you progress.
2. Beginning beginners. Most realistic in the sense of not having predictable ukemi. As you progress you can find that lack of commitment and connection is not an impediment. You capture their balance, regardless of what they offer you, and control the interaction from then on. This also deals with the throwing them without damage despite resistance question. If they feel that they have no choice but to fall, even without any wrist lock pressure, they will not question realism. Pushing them to learn connection is still necessary, as it is necessary for their training, not to make your techniques work. Of course this assumes you are not a beginner yourself, as two confused people provides four times the confusion as one.
3. Non beginning beginners. Brown belts are such fun. Great for some occaisional rock and roll, and they even thank you.
4. Peers. Great fun to just play, but less valuable for real training, except for occaisional mutual figuring out what the Shihan just did type analysis.
Regarding teaching beginners to take atemi seriously, in 90% of cases, an amiable question about whether they really want to practice learning to let someone hit them will make them start dealing with atemi. It is, after all, such a reasonable question. For those that require something physical before taking atemi seriously, stop your fist close to their face, make light contact and give a light shove with your knuckle or knuckles. Target areas contain enough nerves to make that a bit unpleasant without being cruel or with any chance of bloody lips or nose. An amiable cheery "you really don't want to let people be able to do that to you" makes the medicine go down nicely, as it is clear you are really trying to help.