Your scripture above is inapplicable to Marian doctrine, which is one reason why your former church advises extreme caution when interpreting the word. Contextual, linguistic, historic and other nuances play a huge role.
Your posts from Matthew for example, here is more:
Matthew 6
6 But thou when thou shalt pray, enter into thy chamber, and having shut the door, pray to thy Father in secret, and thy father who seeth in secret will repay thee. 7 And when you are praying, speak not much, as the heathens. For they think that in their much speaking they may be heard. 8 Be not you therefore like to them for your Father knoweth what is needful for you, before you ask him.
As you can see repeating a prayer wasn't the issue, "showboating" piety was.
1 Tim 2:5
5 For there is one God, and one mediator between God and men, the man Christ Jesus;
The Father chose His Son to be the mediator, and I want to be sure to use His structure, not mans. No Saint Paul the mediator, or Mary the mediator. One mediator between God and men, the man Christ Jesus; (no one will lose their salvation over misunderstanding this point, only relinquish blessings and power they have coming to them as a believer).
As you can see repeating a prayer wasn't the issue, "showboating" piety was.
I agree, Jesus was making that point as well.
Matt 6:7
7 And when you are praying, speak not much, as the heathens. (emphasis added)
You keep missing the "speak not much" part. Psalms 119:19-16
9 How can a young man cleanse his way? By taking heed according to Your word.
10 With my whole heart I have sought You; Oh, let me not wander from Your commandments!
11 Your word I have hidden in my heart, That I might not sin against You.
12 Blessed are You, O LORD! Teach me Your statutes.
13 With my lips I have declared All the judgments of Your mouth.
14 I have rejoiced in the way of Your testimonies, As much as in all riches.
15 I will meditate on Your precepts, And contemplate Your ways.
16 I will delight myself in Your statutes; I will not forget Your word.