Can a Large creature move past a Small and a Medium creature simultaneously?

Question

A halfling and a human are blocking a group of winter wolves (size Large) on a 10-foot-wide ledge. Can the wolf move past the blockers by squeezing or overrunning:

• Can the wolves just squeeze through the halfling's space since it is two sizes smaller? Could they do this at 1/2 speed or 1/4 speed?
• Do the wolves need/get to overrun both at advantage, since they only take up 2 squares and winter wolves take up 4?

(We're playing G2, "The Glacial Rift of the Frost Giant Jarl", the second module in Against the Giants from the Tales from the Yawning Portal book.)

Answer 1

It's not moving past, it's moving through at 3 feet for every 1 foot

It's a bit weird to imagine, but you've basically described that there is a Small and Medium creature 'blocking' a 10' wide space.

The rules around moving through other creatures are found in Chapter 9 of the PHB under Movement and Position

You can move through a nonhostile creature's space. In contrast, you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature's space is difficult terrain for you.

So, while each creature controls a 5' space, there are specific rules about moving through. And if the space that a creature wants to get to means you have to go into another creature's space, then the above rules apply.

In your case, you've got a medium and a small creature holding the line. As they are hostile to the winter wolves, the spaces can not be moved through unless there is a size different of two sizes.

The medium creature does not allow movement through, as they are just one size larger. But the small creature does allow movement through, as they are two sizes smaller.

Passing through the line

If the winter wolf wants to get 'through' the line, they must pass through the space of the small creature and treat the movement as difficult terrain. However, that space is too small for them, so now the squeezing rules come in. Those are found in the same section and state:

A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it's in the smaller space.

This means that the winter wolf must treat passing through the space as difficult terrain and are being squeezed. So each foot of movement actually costs 3 feet of movement (an extra foot for difficult terrain and an extra foot for squeezing through.)

Also note that if they leave the reach of either creature, the winter wolves will also trigger an opportunity attack.

A second option is in the DMG: Overrun

Chapter 9 of the DMG also covers some additional optional actions. If the table/DM decides, there is another option for moving through hostile creatures which opens it up beyond the above and ignores the size differential requirements in exchange for additional mechanics:

Overrun

When a creature tries to move through a hostile creature’s space, the mover can try to force its way through by overrunning the hostile creature. As an action or a bonus action, the mover makes a Strength (Athletics) check contested by the hostile creature’s Strength (Athletics) check. The creature attempting the overrun has advantage on this check if it is larger than the hostile creature, or disadvantage if it is smaller. If the mover wins the contest, it can move through the hostile creature’s space once this turn.

Answer 2

It a question of Interpretation of the Squeezing rules

Concerning your first question, the rules are very clear, see PHB, p.191, if you don't use squeezing:

A creature's space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions.

If a medium hobgoblin stands in a 5-foot-wide doorway, other creatures can't get through unless the hobgoblins lets them.

These rules suggest that two halflings/humans can control a 10 foot ledge and that the wolf cannot pass without a special strategy unless the blockers let it. The wolf could pass through the halfling's space but not the human's.

The rules on squeezing (PHB p. 192) state:

Thus , a Large creature can squeeze through a passage that's only 5 feet wide.

It is not entirely clear if this applies only to actual physical width of spaces or if the wolf can willingly squeeze itself together to avoid the human's zone of control. This is up to the GM.

The speed for moving through would be 1/3. Moving through another creature's space always counts as difficult terrain. Both the rules on difficult terrain and on squeezing state that the movement costs one foot extra per foot moved. Therefore, each foot would cost two feet extra and the total would be three feet,making speed one third of the normal.

The human needs to be overrun, which has advantage. As stated above, the wolf would need to move through the space of both defenders. Successfully overrunning a creature (DMG p. 272) allows moving through that creature's space once on the same turn. Since the wolf is allowed to move through the halfling's space anyway, only the human needs to be overrun to be allowed to move through their space as well. Doing so has advantage since a human is Medium which is smaller than Large. Moving through the defenders works at half speed. Their spaces are still difficult (even if the wolf would also overrun the halfling which is unnecessary), but squeezing is not necessary in this case.