With the longitude example, you commit the error of mixing up axes and spatial position. The longitude fixes the rotational position about the axis of the Earth, from an arbitrarily chosen reference. The value of that rotational position is constant between the poles along the parallel (coplanar) to the axis of the Earth because you have not changed your rotational position which the longitude defines. This is nowhere applicable as an analogy to the actual point we are discussing - which is picking a moment to begin something, when you don’t have Time as a fundamental dimension to define that beginning moment.
If the Universe was a pinpoint of say 1/1000th cubic inches spherical volume, what was present to define the boundary of this pinpoint radially away from the centre? More space? A mirror?
Are you saying that if nothing changes, Time stops? If so, when exactly does Time start to flow?
Yes, it must be so. Time is the measurement of change.
If the universe is a void, or a truly atomic particle [classic meaning of indivisible], then there cannot be any time. There can be time with a single non-atomic particle because the subcomponents can change in relation to each other.
If so, when exactly does Time start to flow?
Easy, when there are multiple items in existence.
Your question is kind of analogous to software in computer programming; in specific a variable coming into existence.
Yes, the analogy is flawed in that it's time-based, but the issue of scoping is still relevant: the creation of something that wasn't there before.
This is what happened when God started creating and said "Let there be light", bam! multiple particles, and therefore time.
That they suddenly came into being is not doubted, we're here after all. I'm not sure what it's like to be outside of time, I am after all wholly bounded by it and therefore have only time a a point of reference... but such a concept is not unheard of in the Bible, in Hebrews the prophets and heroes of faith are said to be Justified by faith looking forward to Jesus, but if God is outside of time then Jesus on the cross and Abraham trusting God are as you put it "the same moment" from God's perspective then of course Jesus can save those who died before he came to Earth... and likewise that everyone born after his death can be forgiven means that his death did not just cover sins up to that point. God is the God of Time, and not even Time can stand in God's way preventing Him from accomplishing His purposes.