Hi there Bradley-san.
As you may already know, を marks the object of the sentence, while は marks the topic (or sometimes subject) of the sentence.
For the first 2 examples, in the first one, すし is the topic of the sentence (what are we talking about - sushi), so は is appropriate. In the second one, the topic of the sentence is understood from the context (we/ you + someone else/you + a group of people), while すし is the object of the sentence (what is going to be eaten) -> therefore, を is used. A more complete version of the sentence would be: わたしたち は おすし を たべ に いきましょうか？
Now let's take a look at the third example: You use は on すし, which implies that すし is the topic of the sentence. Here, it sounds as if you're inviting the sushi to go eat with you :). Here, you are also part of the topic of the sentence, because even if you didn't mention yourself, it is understood from the fact that you used the "invitation" form of the verb いく (to go).
As for the last sentence, it would be more fluent to use けど for "but" (and you lost an い at the end of the むずかしい): むずかしいですけど、すきです！
Hope this helps :),