Yes you can, I’m on mobile right now. Just open Chrome or Safari (or whatever browser you have) and go to the forum. You will have to log in if you haven’t done this before.
Yes of course there is. Go to army management, click on the unit you want in your defense, and select the blue button that puts it into your defense army:
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You cannot, to my knowledge, get push notifications on your phone from the forum. But whenever someone likes or replies to your forum posts, mentions you in a post, or starts following you, you receive a notification on the forum (a red exclamation mark by the bell symbol on the bottom right). Simply click this, and you can visit the thread or profile involved.
This is where the bell is located:
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