I agree with not holding the infant until they sleep, that probably makes the greatest difference. I don't understand the check-ins, because they fall asleep so quickly anyway and we don't want to disturb them.
There is nothing inconsistent about holding an infant and then sleep-training a 1 year old.
Now that she's older and speaking, she sometimes doesn't want to go to bed and will tell you. Then it helps to communicate that you'll be back and they're not on their own. Gives enough comfort that I almost never do more than come back once after 10 minutes or so, and over half the time she's already sleeping. If not I just say good night again, hand back the stuffed animal that by that point has usually been thrown out of the bed and that's it.