Since WHMCS Services now has several active versions and also different types of updates, there are 4 types of updates. These are:

Major Release: A major release is a new main version and contains changes to the core and usually introduces new features. 1.0 and 2.0 are major releases. You can recognize one such an update also the first number of the version number changes. With Major Releases there is always only one complete upgrade and no incremental.

Minor Release: A Minor release includes new features and can also include changes to the schema and prepare major updates. The versions 2.1.0, 2.2.0 and 2.3.0 are for example Minor Releases. These updates always change the average number. With Minor Releases there is always only one complete upgrade and no incremental.

Maintenance Release: Maintenance Release is, as the name suggests, maintenance updates where bugs are fixed. The versions 1.3.1 and 1.1.2 are, for example, such updates. Here the last number always changes. There is also a small incremental update for maintenance releases.

Security Release: Security releases contain security-relevant updates which, for example, close security gaps. These updates are usually accompanied by maintenance releases and should be implemented promptly. Here again, the last number of the version number changes.

Depending on what it's for you can add it as a link in:

/templates/template_name/includes/menu.tpl for the main menu or the following for the top right toolbar:

/templates/template_name/includes/toolbar.tpl

<li><a href="{$WEB_ROOT}/index.php?m=loginverifypin">Login verify</a></li>

Hexa Extra Template 
You can edit header.tpl file (header.tpl.new can be renamed to header.tpl in overrides folder). Overrides folder will help with keeping customization during updates.
The navbar li item can be duplicated and customized for your needs.

<a href="/index.php?m=loginverifypin">Login verify</a>

Version 4.5.x you don't need to edit any template you can do of the Hexa Extra Menu

Flattern Extra Template

You can edit header.tpl file (header.tpl.new can be renamed to header.tpl in overrides folder). Overrides folder will help with keeping customization during updates.
The navbar li item can be 

<li><a href="index.php?m=clientnotifyn">Notification Settings</a></li>

Depending on what it's for you can add it as a link in:

/templates/template_name/includes/menu.tpl for the main menu or the following for the top right toolbar:

/templates/template_name/includes/toolbar.tpl

<li><a href="{$WEB_ROOT}/index.php?m=clientnotify">Notification Settings</a></li>

When the website goes down, does the system automatically recheck to confirm before sending an alert to the customer?

ANS: Yes, if the website is down then it will check other locations to be sure that it is offline. If it is offline on all locations then it will report the monitor as DOWN otherwise if even 1 location finds it up then it will report it as ONLINE.

You can define one-time and recurring maintenance windows to pause uptime checks. We will not send you downtime alerts on planned outages.

Blacklist (DNSBL) Monitoring service will ensure the IP of your mail server has not fallen into one of the major DNSBL (DNS-based Blackhole List) blacklists.

Email blacklist check is vital for your business because if your server is blacklisted, spam filters might block some or all of the emails that your servers send out.

One the monitoring servers, do you need any software to be installed? What OS and server specs must those monitoring servers contain? What if the server is being overloaded, how will i know that? 

ANS: The system is built for Apache and is tested on cPanel with Apache. I do not believe that it matters which version of Linux OS you have installed. I am unable to recommend a specific hardware specification as we have not had any reports of any of the monitoring services over-loading any servers. An average medium VPS should be enough for your WHMCS + The monitoring module. You do not need to install any further software presuming you have PHP and MySQL Installed as required by WHMCS.

If this was monitoring 1000 sensors, how much traffic will it generate? How much CPU will it use? How much memory will it use?

ANS:  In server monitoring we have some solution to check from multi locations and some times it's taking while to finish in all location then we recommend don't use lesser then 5 minutes monitors to all can working right, about the CPU it's don't cost many process

If this is hosted on a cPanel server with 50 other sites (where WHMCS is hosted) how much impact will it have on the other sites if CloudLinux is not used?

ANS: there is no problem to you use in shared hosts but we recommend to use in VM or dedicated, everything about the processing time and how much servers need to monitor, I can tell if you have many monitors you time is important and other is OK