Bit unfair to compare W&K and H&M in terms of PDA. Noone can be really prepared for that role I'm sure, but Kate had a much longer lead up.
For a start she went to Marlborough, a leading public school where she hobnobbed with aristos, including some of the Princes' friends and others of the upper class. So she was already comfortable socially with that world and familiar with their interests.
Then again at St. Andrew's, she and William were afforded a unique time for a few years where the press left them alone. So they have grown into young adults together and I think knew each other for about a decade or at least 8 years before they announced their engagement.
So Kate had a much better handle on what the British public expected of her and also saw behind the curtain somewhat (deliberately on PW's part to make sure she was aware of what she was taking on).
I think a PP is right that all Meghan would be able to compare it to would be American celebrity. Although she did some work for the UN she was not involved with politics so understandably hard for her to have a clear conception of expectations.
I'm sure she is a good actress and is a nervous wreck a lot of the time, as all eyes are on her and very quick to judge and criticize if she gets it wrong.
She and PH are in the honeymoon period of their relationship and they are clearly strongly physically attracted. I know they are getting a lot of criticism for holding hands and her grabbing his arm but she is suddenly in this very public role and it seems to reassure them both and help them get it right. It will probably be toned down over time as her confidence grows.
I really like Kate but I also know she is a very British gel who understands what we all find acceptable. Meghan doesn't have that cultural knowledge and coming from a show biz background might judge it wrong sometimes. To be fair to her she really has not had a major mishap and did a cracking job on the tour.