Firstly, the Mlem app looks amazing and works brilliantly. Well done to all involved getting it to this point.
I'm glad you like it :) You're correct that Mlem doesn't include these features yet.
First is the ability to see the number of existing crossposts in the post feed.
This is tricky to implement at the moment - we'd need the Piefed developers to make some changes to make this easier for us. When we ask the PieFed server for a feed of posts, it doesn't tell us the crosspost count for each one. To get that information, we need to ask the PieFed server for each post's crosspost count individually, which is inefficient and difficult to manage. I'll raise an issue with the PieFed developers and see if this is something that can be improved upon 👍
Second is the ‘Notify about replies’ feature
This is high on the list of new features we'd like to add, but it will require quite a lot of code rewriting on our end so I can't make any promises on when this might be done. You could consider using Interstellar if this feature is highly important to you; according to this page Interstellar is currently the only app that can do this.