[RPG] How does the spell Slow affect freefall

Assuming the PC has successfully rolled below an 11 on the d20 to cast a spell, the spell takes their action on this turn. If it successfully dispels the spell, all effects of the spell are also removed.

An affected target's speed is halved,

This ends as soon as slow is dispelled. Since movement says you can move up to your speed, you are no longer impinged by this effect and any movement you might have left using your full speed can be used.

it takes a -2 penalty to AC and Dexterity saving throws.

This ends as soon as the spell effect ends.

it can't use reactions.

This ends as soon as the spell effect ends. Note that it doesn't say the PC doesn't have reactions, just that they can't be used while under the effect.

On its turn, it can use either an action or a bonus action, not both.

The PC has used an action on its turn to cast dispel magic. Assuming the spell is successful and it dispels slow, and the PC has a bonus action to make, the PC can now take that bonus action.

If they didn't roll below 11 then they are still affected by the slow until their next turn when everything stated above applies. In between these turns they are still casting the Dispel Magic which requires concentration: if that concentration is broken before their action on the next turn the Dispel Magic is lost and they remain slowed.

Yes, it should be able to

Consider it from a practical perspective: if something is gliding (not being propelled), is it moving under its own power really? It doesn't have a flying speed, because it's not really "flying". I would rule that it doesn't consume movement at all, since the only remaining speed (land) certainly doesn't apply here. You're really just steering.

See the spell feather fall:

Its descent slows to 60 feet per round until the spell ends.

Feather fall probably a good place to start, as this spell deals with slow falling and it lists a definite speed for your fall. Other than this, the other source for a falling speed I can find is the XGTE rule (500' per round during free fall). I think it's fair to rule that Manta Glide slows your speed to 60' per round similarly to how feather fall does.

Like this, it would take only one round to finish falling for 50', and during that round you could glide 100' horizontally. It would of course take more than that if you fall from say 120' feet (2 rounds, 240' horizontal motion total, 120' horizontally per turn)

This would be a relatively quick fall, but likely not fast enough for any other characters or enemies to have issues targeting you, if they can reach that high in the air. Your DM is free to make rulings on the specifics of how far you fall per turn, but 60' per round is probably a good starting point.