Geordie: OK, you do have a very good understanding; I guess I mininterpreted. Now, I’m not sure I get your original question: If the ger/BT learned from a Rabbi in the Yekkishe shul, then he would take on that rabbi’s minhag (wrap tefillin). If he defaulted to the M”B, he would wrap tefillin without the brachah like the M”B says. Was the question just regarding the brachah?
On a side note, 3 hours is a bigger problem given that it’s only based in family tradition rather than having a source in the poskim.