jQuery AJAX requests - Plugin to Backoff on failure

I'm brand new here, but I've read the guidelines and looked at some up-voted questions. Hopefully I understand how I'm supposed to do this... if not, please let me know and I'll fix my question.

Explanation

I have written a simple javascript plugin of sorts. It takes the settings object for a jQuery.ajax request and a few options (frequency, failure tolerance, etc) and then periodically issues the ajax request. If the request fails, it increases the interval to the next request. If success follows a number of failures, the interval is slowly reduced to reach the original frequency.

Usage looks something like this:

var ajax = {url: 'example.com', success: mySuccessFunc};
var cw = Coward(ajax, {}); // Request my url with default settings
cw.Start();
... // Coward is running, refreshes happening periodically 
cw.Stop();

Purpose of Review

I'm open to any feedback about my code that you're willing to give me. However, I came here seeking a few things specifically:

• Are there any bugs that will result in an excessive number of requests or other undesirable behavior? Am I doing anything unsafe? Race conditions?

• JSLint warns me about scope in a few places, but the code works. Am I violating standards? Should I be handling scope differently (particularly with the 'underscore' functions and the 'live variables')?

The Code

``
// Coward - One time or periodic refresh that retreats if anything goes wrong.
//
// This tool was designed for use with jQuery 1.9 or later
//
// Written by Adam Jensen

function Coward(userReq, custom) {
'use strict';
//==============================================
// Options and variables used by the coward
// - Do not change opts` after invocation -
//==============================================
// Combine user options with our defaults
var opts = $.extend({
frequency: 1000, // How often to issue ajax request
ceiling: 10, // After this number of failures, multiple will not be applied
mult

You should check that the supplied success and error callbacks are functions not just that they exist.

if (typeof userReq.success === "function") {
    var successCallback = userReq.success;
    delete userReq.success;
}

The way you have it now, if a user uses the following configs :

var ajax = {url: 'example.com', success: null};
var ajax = {url: 'example.com', success: "my success function"};

will cause the check below to pass, resulting in an error

if (successCallback !== undefined) {
   successCallback(data, status, jqxhr);
}

StackExchange Code Review Q#106662, answer score: 2