As a Protestant I am increasingly sensitive to a growing trend where Protestant authors are advocating an exploration of the "deep well" of Christianity. The idea is we can become more authentic and mature Protestants by drawing on the rich heritage of Christianity as a whole.
It might be easier for others to address the philosophy of the authors you mention if you would name them and some of their writings.
What increasingly convicted me is that Orthodoxy is a complete package - you can't take practices out of it and expect them to really bear substantial fruit in fact, I am convinced, that Protestants experimenting with practices like the Jesus Prayer and cyclical fasting outside of the liturgical and confessional context is nearly impossible.
This is far from clear. Fasting as an individual spiritual discipline has been part of Christianity from the very beginning (e.g. Acts 13.2), long before the Byzantine rite reached its present form. Why do you think the one is inseparable from the other?
What also really struck me is the issue of discernment . . . Do Protestants expect to be able to successfully discern and explore all these myriad strands of spiritual and devotional life guided simply by the Holy Spirit? Can even a small gathering of Christians expect to travel this road safely?
Who says they can't? Your words are so broad that they can cover a wide range of practices, some well-advised, some ill-advised.
Some further remarks:
(1) Straw-men and sweeping generalizations about others' traditions have no place in discussions of what constitutes a good approach to fasting. A believer's prayer-rule or life-rule is normally a matter of individual conscience. There is no one-size-fits-all rule. "Let no one pass judgement on you in questions of food and drink or with regard to a festival or a new moon or a sabbath." (Colossians 2.16). If we are to let no one pass judgement on us, it is reasonable to suppose that we should not pass judgement on others. If a vegetarian friend, who normally eats no meat, adopts the spiritual discipline of eating meat on Wednesdays and Fridays in Lent, we have no basis for finding fault. Of course such matters cannot always be left entirely to the individual, but this precept should not be an excuse for meddling. If someone's fasting veers off into anorexia or other dangerous ways, his relatives, friends, and pastor should intervene; others should ordinarily mind their own business.
(2) On the particular matter of the Jesus Prayer, which you mention: I do not use it, so everything that follows comes from a stranger to the technique. If I were nevertheless in a position where advice was demanded of me, my normal caution would lead me to advise that no one should adopt this technique except under the tutelage of someone who is known to be discreet, reliable, and spiritually mature, and who has many years' experience using the same technique. Such people may be rare, but I see nothing that makes them inherently inseparable
from the Julian paschalion.