There should be many way to do what you want to do, and your description is way too short for giving precise answers. But there is some sugestions 
Some question : what is a combo for you? Is this just more than one attack following each other? Does making a combo give bonus? Is there something else?
Combo
To create a combo (understand : a skill) I would proceed the same as I did for my spell system. Create a Combo class and implement all the function you need. (a Damage function, heal, stun, etc…) And also the variable (power, cost in mana, chicken, etc…)
Once this is done, go in your editor and create a Blueprint based on the Combo class and just add the function you need, set the parameter and voila, done.
Chaining Combos
I would add to my Character a container in which you store all the combo that the player input. And once the player validate/the time is up/the combo limit is reached the Character start executing each combo in the order.
If you want to check if the combo in the container correspond to a special pattern, implement a function which will take into its parameter a container of combo name and simply check if this container is the same as the one in the character. You will have to call this function just before executing the combos in your blueprint.
Mmh, depend on what are the action your are speaking about? Thing like attack, use an item, help a friend, etc? I need more information here ^^’
Intredasting, this is topic I didn’t think of for now, but I will certainly need to implement one, someday.
Can you add a “wait for input” node in a blueprint? If yes, then the conversation blueprint will be a chain of : “Draw text” -> “Ask for choice” -> wait for input -> “switch which call the correct node depending on the input” -> “Draw text” -> etc…
Sorry, there I’m totally ignorant, so I don’t have ,any advices here ^^’
://wiki.unrealengine.com/Basic_Inventory_/_Item_system
That could help you start, maybe 
I hope that I, at least, gave you some ideas