>>625126
Good question OP. I think the difference is the motive behind the action. When people pray in public, like Jesus mentions in Matthew, they do it to as a way to show how better they are than those around them. To put it simply, praying in public is like someone announcing "I give 100s to kids in Africa"
Telling someone you'll pray for them, however, is usually used as a way to show consideration for others. When this happens, they are not saying they are better than the person for whom they pray for.
I think your last example may be committing the sin of your first point. People that respond with "I'll pray that you leave your misguided ways," are making intercession into a passive-aggressive action, rather than a benevolent one.
Lastly, if you do encounter people like that, realize that Christianity is more than backhanded insults. Also–and I imply no insult when I say this–they probably aren't prepared to defend the faith.