No. You are failing to understand the problem. I think the Lord’s Prayer and the resurrection appearances are fundamental to Christianity. I think you cannot come up with a reason why Mark would leave them out that does not also raise the question of why he bothered to write a gospel at all.
I'd say that Mark includes the most important elements of Mt;s and Lk's gospels, this being evident from comparison of sections of Mt and Lk where Mk only has a summary statement including the key elements, such as Mk 13:32-33 // Mt 24:36-44.
All the elements of the Lord's prayer are included at various points in Mk. Eg in Gethsemane Jesus addresses God as Father, prays for God's will to be done, and tells the disciples to pray that they will not be tempted. After the fig tree withers, Jesus tells the disciples to pray with faith for the removal of a mountain, figurative for earthly obstructions to be removed to make way for God's kingdom to come. When clearing the temple, Jesus refers to Jeremiah 7:11, "Has this house, which bears My name, become a den of robbers in your sight?" (hence, Hallowed be thy name).
In the same passage Jesus tells us to forgive anyone we hold a grudge against, so that we may also be forgiven.
At the feeding of the 5000 Jesus teaches the disciples to rely on God for food (Mk 6:52).
If Mark omitted Mt's Sermon on the Mount (and other long teaching sections in Mt and Lk) for brevity, then he would naturally omit the Lord's prayer, and not be concerned since it was available to read in his two sources anyway, and also indirectly in his own gospel.
Mk doesn't have the virgin birth either. Nor the ascension (but then, Matthew omits that as well).
It is possible to say that Mk included only the material that was important for evangelism. The cross and resurrection are the fundamentals of the Easter story, and both are reported in Mark. Given that Mark is in a hurry, we should not expect to find all the information shared by Mt and Lk.
Jesus' post-resurrection appearances are presupposed by the angel's instructions to meet him in Galilee, as well as by the raising of Jairus' daughter (names of key eyewitnesses included).
It's possible that Mark was intending to focus in on what Jesus had just done ('wrought salvation in complete isolation' - Rosenstock-Huessy). This is amplified by the way Mark leaves us with no indication that the disciples understood yet who Jesus is (the Son of God) - unless the long ending is authentic.
Mk 16:8 ends with the Greek word meaning 'for'. This is strange as it implies there is more to follow, but given Mark's style it can be understood to be the original wording. It is a similar situation to when Joseph identifies himself to his brothers in Genesis 45:3, where in the LXX the same word order is found ('gar' at the end of the sentence).
You also fail to address the other reasons for Markan priority.
Ok, I will address one now: The 'spitting miracles' are not in Mt or Lk. You have said that Mt and Lk considered these miracles inappropriate, but equally it is rather unlikely that Mt and Lk, copying Mk, would both independently decide to omit them both, but include all his other healing miracles.
Let me know which others you feel need addressing.