The context in which we should understand this part seems to me to be clarified by a remark Allan Wolter makes in the Preface to his edition of Scotus' De primo principio, namely that this latter work "may be the most carefully thought out attempt of any schoolman to prove the existence of God within the epistemic norms for demonstration laid down in Aristotle's Posterior Analytics." Anyone familiar with that work will appreciate the justice of that remark, as well as the nature of Scotus' project in proving God's existence and attributes. For the subtle doctor is not satisfied with providing "reasons" to accept God's existence or "ways" by which it can be proved; he wants to provide a really rigorous demonstration, one which if properly grasped will give the human mind certain knowledge.
The fifth part here is jumbled. It is clearly divided into three parts, but the parts seem to have been mixed up, with the apparent beginning only coming in around paragraph 70 in the critical edition, and the final third seeming to be put first. Paragraph 70 is where I start.
Assumption 1: In essentially ordered things one must posit a first, which is unique in that ordered series, and cotemporal with it.
Assumption 2: There is an essential order in every kind of cause.
These are called "assumptions" here, because the ability to prove them is precisely what is called into doubt in this part, although it is on these principles that, for Scotus, natural theology--knowledge about God which cam be proved from natural things and insofar as he is the cause of things known to us--rests. Of course they are not "assumptions" for Scotus absolutely, because he spends a lot of time in other works trying to prove them. But here he says, "These two propositions are assumed, of which the first has three parts [i.e. that God is 1) first, 2) unique, 3) cotemporal with his creation], [but] the second is simple. But although either of these parts, namely 'first', 'unique', [and 'cotemporal'], may be probable, still it would be difficult or perhaps impossible for us to prove it simpliciter by necessary argument and by purely natural [reason]."
Conclusion 1: In the genus of efficient cause one must posit a unique first efficient [cause], which exists now in the nature of things.
This follows from the two assumptions, and this cause is called God.
Conclusion 2: Every efficient [cause] is more perfect than its effect or equally perfect, because nothing acts to a greater extent than it is in act.
Conclusion 3: God is more perfect than every effect.
From Conclusions 1 and 2. If God is the cause of everything else he must be at least as perfect as everything else.
Corollary: And so [God] is the most perfect of all and the highest in every difference of being, which simply implies a perfection, among which are one, true, good, necessary etc., because whatever is such is simply a perfection in some way . . . Put here the boundary of wha tis knowable about God by natural necessary reason; and this is supposing those first two assumptions.
This is classic scotism, showing that natural theology is developed by showing that God must contain in a supereminent way all pure perfections. But the conclusions are only as solid as the ground they're built on, and Scotus now begins to explore the difficulty with providing true demonstrations of natural theology's foundations.
How can the first part of the first [assumption] be proved, namely [that God is] "first" more in essentially ordered causes than [in] accidentally [ordered ones] [since in accidentally ordered causes there is no first cause simpliciter]? How can the second part by proved, [that God is] "unique"? How can it be proved [that God is] "cotemporal?
A primary difficulty that Scotus brings up here is how can it be proved by demonstrative arguments that the God who created the world--the temporally "first" cause--is the same as the principle which stands at the head of the chain of essentially ordered causes in the world now? How do you prove that the creaturely order of secondary causes need to be conserved in being by the first cause, even after their initial creation? Scotus sees that the Big Bang does not prove that God exists now, but only that there was an first cause in the sense of an initial one. Our impulse, of course, is to appeal to God's eternity or immutability, but if the foundations of natural theology haven't been secured yet, we can't appeal to its posterior conclusions. And if we can't prove that the initial cause is identical with whatever is the current "first" [in the ontological, not the temporal sense], then what happens to our natural certainty that God is "one" and "unique"?
The second assumption does not seem to be proved necessarily. For if many effects are so coordinated among themselves, so that none of them has the character of an effect with respect to another--as with a cow and an ass--why are all causes so ordered, that the first [in one causal chain] is always the cause of another [causal chain]? If this name "God" is given to some numerically identical first efficient [cause], it follows from this that it cannot be proved that God exists in the real world now, because if he no longer existed the coordination [of causes in the world] would remain through another [first cause, another God] univocal with him.
Again, remember that Scotus has not yet made his argument for the incompossibility of two first causes, because in this work the conceptual apparatus with which to do so hasn't been developed yet. From comments he makes later on it seems that what Scotus is showing what kinds of problems a thinker proceeding along Aristotelian lines, with principles and arguments from the Physics, is going to run into.
Beyond this, Scotus continues, the second assumption does not prove that any God exists, even a new one like the first, if it cannot be proved that conservation of the created order is needed as much as the original act of creation. Otherwise we can only conclude that a first cause is necessary for the world's becoming, not for its being. Whence it follows only that either exists or did exist once, as from a house it can be proved that a builder exists or did exist once.
Therefore these things, which it seems cannot be proved by necessary arguments from merely natural [reason], are laid out in order in the conclusions, as well as some others which cannot be proved.
That is, if these primary propositions of natural theology turn out to be unprovable, so are whatever less know propositions which follow from them and depend on them.
I omit any detailed discussion of the rest of Part V, in which Scotus lays out the "unprovable" conclusions systematically. It's rather horrible reading, a kind of anti-Contra Gentiles in which Scotus insists at great length that unless he who builds metaphysical systems builds his house upon the rock, he labors in vain that builds it.
In any case, the problems that he lays out here are not solved here. The reader must go to the De primo principio or other theological writings to see how Scotus deals with them. As I said yesterday, what seems to emerge is that the Theoremata is a kind of testing-ground where Scotus is working out the consequences of different approaches before giving them a more authoritative treatment elsewhere.