Regarding the event_data issue: your JSON is inconsistent. In one case it's an object containing the user_agent string:

"event_data": {
                "user_agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/45.0.2454.101 Safari/537.36"
            },

And elsewhere it's a list of quiz_data:

"event_data": [
                {
                    "quiz_question_id": "220",
                    "answer": null
                },
                {
                    "quiz_question_id": "221",
                    "answer": []
                } [...]
            ],

This wouldn't be picked up by a JSON linter, because it's just checking that the data is valid - not that it's consistent.

I notice that the parser code generated by JSON2Apex is different from your parser code above, and e.g. creates different classes to handle the different types of answers and event_data:

//
// Generated by JSON2Apex http://json2apex.herokuapp.com/
//

public class JSON2Apex {

    public class Answers {
        public String answer_exact;
        public String answer_error_margin;
        public String answer_range_start;
        public String answer_range_end;
        public String answer_weight;
        public String numerical_answer_type;
        public String blank_id;
        public String id;
        public String match_id;
        public String answer_text;
        public String answer_match_left;
        public String answer_match_right;
        public String answer_comment;
        public String answer_html;
        public String answer_match_left_html;
        public String answer_comment_html;
    }

    public class Quiz_data {
        public Integer id;
        public String regrade_option;
        public Integer points_possible;
        public String correct_comments;
        public String incorrect_comments;
        public String neutral_comments;
        public String correct_comments_html;
        public String incorrect_comments_html;
        public String neutral_comments_html;
        public String question_type;
        public String question_name;
        public String name;
        public String question_text;
        public List<Answers> answers;
        public String text_after_answers;
        public Integer assessment_question_id;
        public Integer position;
        public String published_at;
    }

    public class Answers_Z {
        public String id;
        public String text;
        public String comments;
        public String comments_html;
        public Integer weight;
        public String html;
    }

    public class Quiz_submission_events {
        public String id;
        public String event_type;
        public Event_data event_data;
        public String created_at;
    }

    public List<Quiz_submission_events> quiz_submission_events;

    public class Event_data {
        public Integer quiz_version;
        public List<Quiz_data> quiz_data;
    }

    public class Answers_Y {
        public Integer id;
        public String text;
        public String html;
        public String comments;
        public String comments_html;
        public Integer weight;
    }

    public class Answers_X {
    }

    public class Event_data_Z {
        public String user_agent;
    }

    public class Event_data_Y {
        public String quiz_question_id;
        public Object answer;
    }


    public static JSON2Apex parse(String json) {
        return (JSON2Apex) System.JSON.deserialize(json, JSON2Apex.class);
    }
}

This is an attempt to handle the inconsistencies in the JSON. I'm not sure whether it'll actually work in practice, but it's worth trying.

If it doesn't work, you have a few options.

• Ideally you would change the JSON to be consistent, but that may not be an option if you're receiving it from a third party.
• Alternatively, you could use JSON.deserializeUntyped() and check the type of each event_data at runtime, but you'd lose the advantages of typed deserialization.
• Or, you could write custom parser code to manually check the value of event_data and parse it appropriately. If you check the Create explicit parse code box on JSON2Apex, you can take the parser code as a starting point.

Not a parser! Just deserialize:

public class Response { final Geometry geometry; }
public class Geometry { final Location location; }
public class Location { final Decimal latitude, longitude; }

Granted, your JSON payload is not valid as currently specified. The above would work though if your structure boils down to:

{
    "geometry": {
        "location": {
            "latitude": 0.0,
            "longitude": 0.0
        }
    },
    "other_properties": { /*values*/ }
}

If you want to copy the payload format listed here, the class would be slightly different.

JSON

{
   "results" : [
      {
         "address_components" : [...],
         "formatted_address" : "1600 Amphitheatre Parkway, Mountain View, CA 94043, USA",
         "geometry" : {
            "location" : {
               "lat" : 37.4224764,
               "lng" : -122.0842499
            },
            "location_type" : "ROOFTOP",
            "viewport" : {
               "northeast" : {
                  "lat" : 37.4238253802915,
                  "lng" : -122.0829009197085
               },
               "southwest" : {
                  "lat" : 37.4211274197085,
                  "lng" : -122.0855988802915
               }
            }
         },
         "place_id" : "ChIJ2eUgeAK6j4ARbn5u_wAGqWA",
         "types" : [ "street_address" ]
      }
   ],
   "status" : "OK"
}

Apex

public class Response { final List<Result> results; }
public class Result { final Geometry geometry; }
public class Geometry { final Location location; }
public class Location { final Decimal latitude, longitude; }