A container class that wraps around `KeyValuePair` to make it "mutable". Also Unity inspector friendly.

KeyValueContainer.cs

/// <summary>
/// A container class which encapsulates the KeyValuePair so it shows up in the editor.
/// Also allows us to update key and value without much hassle.
/// </summary>
/// <typeparam name="TKey"></typeparam>
/// <typeparam name="TValue"></typeparam>
[Serializable]
public class KeyValuePairContainer<TKey, TValue>
{
	/// <summary>
	/// Actual KeyValuePair struct.
	/// </summary>
	private KeyValuePair<TKey, TValue> _keyValuePair;
	/// <summary>
	/// Property to allow access to _keyValuePair
	/// </summary>
	public KeyValuePair<TKey, TValue> KeyValuePair
	{
		get => _keyValuePair;
		set => _keyValuePair = value;
	}

	/// <summary>
	/// Key of the _keyValuePair.
	/// When setting new value it has to create new KeyValuePair and copy over data as we cannot change it directly 
	/// </summary>
	[ShowInInspector]
//			[HorizontalGroup("KeyValue", LabelWidth = 50)]
	public TKey Key
	{
		get => KeyValuePair.Key;
		set
		{
			var newKeyValuePair = new KeyValuePair<TKey, TValue>(value, this.Value);
			KeyValuePair = newKeyValuePair;
		}
	}

	/// <summary>
	/// Value of the _keyValuePair.
	/// When setting new value it has to create new KeyValuePair and copy over data as we cannot change it directly
	/// </summary>
	[ShowInInspector]
//			[HorizontalGroup("KeyValue", LabelWidth = 50)]
	public TValue Value
	{
		get => KeyValuePair.Value;
		set
		{
			var newKeyValuePair = new KeyValuePair<TKey, TValue>(this.Key, value);
			KeyValuePair = newKeyValuePair;
		}
	}


	#region Constructors

	/// <summary>
	/// Convenience constructor that takes KeyValuePair object.
	/// </summary>
	/// <param name="keyValuePair"></param>
	public KeyValuePairContainer(KeyValuePair<TKey, TValue> keyValuePair)
	{
		this.KeyValuePair = keyValuePair;
	}

	/// <summary>
	/// Convenience constructor that takes key and value arguments.
	/// </summary>
	/// <param name="key"></param>
	/// <param name="value"></param>
	public KeyValuePairContainer(TKey key, TValue value)
	{
		KeyValuePair = new KeyValuePair<TKey, TValue>(key, value);
	}

	/// <summary>
	/// Default constructor.
	/// </summary>
	public KeyValuePairContainer()
	{
		KeyValuePair = new KeyValuePair<TKey, TValue>();
	}

	#endregion
}