We can answer these questions based on a diyuk in the language of the Rambam who wrote:
"כל שהוא חייב בערלה יש לו רבעי".
Note - The Rambam didn't say "כל שהוא חייב בערלה חייב ברבעי" but rather "יש לו רבעי". But then the Rambam continued and wrote: "וכל שפטור מן הערלה אינו חייב ברבעי". Here the Rambam uses the terminology of "חייב" and "פטור" and not "יש לו" or "אין לו". From this it would appear that indeed, if the tree is פטור from ערלה it must be פטור from רבעי. For if he would not be פטור from רבעי he would necessarily be חייב in ערלה because the איסור of ערלה stems from the mitzva of רבעי as the Ramban explained. The Rambam cited the psukim in order to show that רבעי is written as a continuation of the mitzva of ערלה showing that ערלה is only a preparation for רבעי. Therefore it MUST BE that any tree that is פטור from ערלה is not חייב in רבעי because if it was חייב in ערלה then it would have to be חייב in רבעי. But this doesn't mandate that any tree that is חייב in ערלה is necessarily חייב in רבעי. Because as we saw - there is ALSO an איסור עצמי - independent איסור, of ערלה, not linked to רבעי.
That is why the Rambam wrote at the outset that since the psukim link ערלה together with רבעי, this means that if the tree is חייב in ערלה then "יש לו רבעי". This doesn't mean that it is חייב in רבעי but rather it means that the tree must have the מציאות that has the capacity to have רבעי. The words "יש לו" mean that the tree COULD have רבעי but maybe practically it has no חיוב רבעי.
So the words of the Rambam are very precise. If the tree is פטור from ערלה it is not חייב in רבעי - if it were חייב in רבעי it would be חייב in ערלה. And if the tree is חייב in ערלה then "יש לו רבעי" - it could theoretically have a רבעי obligation even though practically it might not.
So we have resolved what the Rambam intended by quoting the pasuk and also that there is no question from the fact that according to some only a כרם is obligated in רבעי but other trees are obligated in ערלה alone. There must be a link between ערלה and רבעי [as we see from the pasuk] but that doesn't mean that PRACTICALLY SPEAKING if a tree has a דין ערלה it necessarily has a דין רבעי. There is also no question from the fact that trees in חוץ לארץ are חייב in ערלה but not in רבעי because it COULD have a דין רבעי for it is a tree with fruits. That is what the Kesef Mishna and Rashba meant when they said that the Rambam was only talking about ארץ ישראל but not חוץ לארץ even though the Rambam spoke in general terms. They meant that in חוץ לארץ as well there is a linkage between ערלה and רבעי even though in practice there is no חיוב רבעי.
