Consider the 'hypothetical' situation of a large figure (red filled shape) blocking a corridor as shown. Black lines on either side are walls. A hostile figure (grey filled shape) has 5 movement points; can it still move A5 to A4, then A4 to A3 (since the RRG only says a "figure cannot enter (another figure's) space if this would spend its last movement point")? This does not seem to be explicitly forbidden by the rules, but if it is allowed, what then would happen to the figure with only 1 more movement point and hence unable to move further (as it is wholly within the large figure's space and movement in any direction would require 2 movement points)?

"if this would spend its last movement point" is an incomplete wording (more of an example than a rule really).

You can move through spaces occupied by another figure (enter a space where you cannot end movement) only if you can end movement legally. You cannot voluntarily (intentionally) run out of movement points without being able to end movement legally.

In your example: With 5 movement points the small figure from A5 could enter A4 or B4 (2mp), then return to A5 or B5 (1mp), so it would be legal to enter A4 or B4. But cannot enter A3 or B3, because it would no longer be possible to end movement legally.

I have used the following in the Consolidated Imperial Assault Rules document:

Movement said:

Figures can move through other figures' spaces. The figure must spend one additional movement point to enter a space containing a hostile figure.

When moving through a space the figure can enter the space as long as the figure does not end its
movement in the space. A figure can only enter a space if it can end movement legally.

(ok, maybe the wording could be still be improved)

The problem is that the rules as written prevent entering a space (ie. square) if one would run out of movement points there, but does not prevent entering a space within a massive figure even if it would then result in having insufficient movement points to exit.... It certainly does not seem logical that a figure cannot enter an occupied space by spending its last movement point, but can be allowed to enter an occupied space that it would be unable to leave , but the potential for extreme rules-lawyering is there...

The definition of "moving through a space" is that the figure
can enter the space as long as it does not end its movement
in the space.

If you ignore the rest of the paragraph as an example, the above is all the rule you need. The following chapter reinforces that you cannot voluntarily run out of movement points

· There are rare situations where a figure can lose all of its
movement points while in a space where it cannot end its
movement (such as in another figure's space).

Rest assured that a figure needs to be able to end movement legally to move through other figures. If someone rules-lawyers that, ridicule them for being silly and tell them to ask FFG.