'...Siddhartha Gautama certainly believed that a supernatural world existed. In fact in the Lotus Sutra and elsewhere the supernatural world is said to contain ten directions with a head Buddha associated with each direction...."
Certainly you are right on the worlds depicted in the Lotus sutra. However, these are different from that of Christianity and Islam. The Buddhas of other lands in the ten directions are all emanations of Shakyamuni's own mind percieving the countless galaxies and planets and the whole universe as one giant living entity capable of producing 'Life'(beings) not only our own planet.Indeed, many sceintists believed that there are countless planets in the galaxies exists that are the same with earth. The phrase "ten direction" refers to 8 directions plus the up and down and that means covers the entire universe.
The teachings of mahayana Buddhism are subdivided into two. One that postulates a transcendental beingin which people are taught to believe. These are provisional mahayana teahings, that is, the teachings expounded before the Lotus sutra. Such the kegon sutra, this sutra is said to be first expounded after Shakyamuni attained enlightenement, that expounded the Ultimate truth, the enlightenment of Buddha. According to this Sutra,however, the ultimate truth comes from a unique existence aprt from the phenomenal world. In other words, the sutra says the entire universe including heaven and earth derives fron one Ultimate truth which governs all other existence. In contrast to this teaching, the Lotus sutra reveals that univesal truth pervades the entire universe and is ONE with all phenomena. According to this sutra the ultimate truth is not something confine to a unique existence, and therefore, the universe does not begin with the one truth, not is it governed by the one truth, the truth is ubiquitous in all existence in the universe.
Thus, because of this postulation of transcendental being, provisional Mahayana Buddhism is similar to Chritianity or Islam. However, there are Differrences as well. Provisional Mahayana buddhist sutras teaches us that a Buddha of perfect enlightenment exist in a Paradise far away in the univese and that beleivers can travel to the land of this transcendental being after death.Christianity or Islam teaches that the supeme being transcends our present life, beyond the world and the universe. That is to say in these religions a transcendental being exists outside of the phenomenal world.
In contrast, the Lotus sutra teaches us that instead of transcendental being, the universal self(truth) exist in the depths of our lives within this phenomenal world and the entire universe. In Christianity and pre-lotus sutra teachings this universal self or the ultimate reality is regarded as "personality". The Lotus sutra regards the reality as a "Law"(Dharma)which underlies all phenomena.
...Moreover..."the Buddha’s explanation of the origin of the universe corresponds very closely to the scientific view. In the Aganna Sutra, the Buddha described the universe being destroyed and then re-evolving into its present form over a period of countless millions of years".
All things(the whole universe) reside in the realm of phenomena, subject to the cycle of birth, duration, change and extinction in an everlasting continous cycle of change( as opposed to collapse). The "Expedient Means" (second) chapter of the Lotus Sutra states this principle:
"Phenomena are part of an abiding Law, / that the characteristics of the world are constantly abiding ".
And according to Buddhism, there is no division between physical and psychological aspects of life. The life of each human being is eternal, because it is part of the Universe, which exists eternally . No human being can therefore be created or destroyed. The Buddhist concept of eternity of life is equivalent to the physical law of the conservation of energy and matter, according to which they are never dispersed, but are transformed into different forms. Buddhism furthermore affirms that the Universe has neither been created by an original cause nor moves towards a goal. Due to the capacity of regeneration, immanent in life itself, the universe has always existed.
And you said: " Now we know that the world is flat and its expansion is accelerating so it is most likely to exist forever and expand forever rather than collapse.'
The big bang theory which assumes the beginning and eventual end of the universe In addition that theory, there are different theories which you brought up that the universe is without beginning or end or eternal. And Buddhism takes stand close to that view which I Think you fail to grasp the buddhist principle of "Kuon" or eternity.
Nichiren Daishonin, 13th century japanese buddhist monk, clarifies this concept in his 'Ongi Kuden' stating: "Kuon means being neither created nor adorned but remaining in the original existence".
"Being neither created nor adorned" means that life or the universe itself is not something created at a certain fixed time, but that it exists originally- ever since th infinite past it has been inherent within the universe. "Remaining in the original existence" means that life or the universe itself keep existing eternally. Therefore, in buddhist view of life, it keeps existing from the infinite past to the infinite future, taking the two alternatives forms of reality we call life and death.