First, before I respond on specific points, I'd like you to take a look at my web site:
http://www.thesafemac.com
...just so you can see what qualifications I have for speaking about this topic. I'm not just blowing wind, as many folks on forums can do sometimes.
I was sent a dodgy email to the old iPhone 4 [...], and I clicked on it, had a quick look at it and quickly deleted it.[...] I did not know at the time that just opening them can open the door for malware to install itself onto given device
That is absolutely not true... at least, not using the Mail app on either a Mac or an iOS device. Opening an e-mail message does not do anything that could run unauthorized code on your machine, so there's absolutely no way that any kind of malware could have been installed just by opening and viewing an e-mail message. If you opened a malicious attachment, of course, that could infect your Mac with something, but only if it somehow managed to bypass all of the Mac's built-in security. On an iPhone, the only code allowed to run must be downloaded from the App Store, so it's not possible for any code attached to an e-mail message to run.
I reached over to phone and when I picked up I saw that it was asking me for my Wifi password. [...] I checked and it was Wifi was off, but when I tapped Wifi and opened it, underneath it was revealed that it was actually on - same with Bluetooth etc.
Sounds like a glitch with the phone. That's definitely not evidence of malware.
the person had half an hour to crack my parents' home 10 character Wifi network from the Iphone 4
If your wifi network actually got hacked (you don't say how you know that it was), it is probably mis-configured to allow remote administration with no password or with the router's default password. That definitely wouldn't be something they did via the iPhone.
The person even taunts me that they can see exactly what I am doing on them - they can see all my pictures; wallpapers; know what online radio stations that I listen to; read my emails; watch me playing video games; and know everything I type in my browser; and what I don't type as well via screenshots. I know this because they confirm to me.
There are a few possible explanations for that. One is that this person has gotten access to some online account of yours that gives them access to your machines. For example, if your Apple ID/iCloud account is hacked, depending on the settings on your computer and how you use iCloud, that person can see a lot of your data, locate all your devices and even potentially get access to your Mac via Back to My Mac.
If you use other remote access software, such as LogMeIn, that would explain the issue on all the machines. The LogMeIn account (or whatever other remote access account you have) has been compromised.
It's also possible that your hacker has somehow gotten access to your network such that he/she can see all the network traffic being sent and received. This could expose passwords and other sensitve information that could lead to gaining access to cloud-based data or accessing your computer via some remote access software that you have already installed.
Another possibility is that this person has a local accomplice who is able to get physical access to your machines, and who has installed remote access software of some kind. With physical access, a knowledgeable person can do a lot!
What is really not a likely possibility is for all these machines to be simultaneously and transparently infected with malware remotely. Each device would have to be infected with something completely different, through different means, and each of these pieces of malware would have to get past the protections running on each system. (To see more about what protection is built in on Mac OS X, see How does Mac OS X protect me?.)
I hear what you say about the iPhone, but I have been reading news articles online recently which actually claim it is the least secure phone.
I'm not sure what news site you were looking at... probably not a reputable one! Android has gotten a reputation for being the most insecure system on the planet, while iOS is known to all but a few biased detractors as being quite secure. Take a look here, for example:
http://thenextweb.com/google/2013/08/26/internal-us-government-memo-warns-author ities-about-android-malware-threats/
Fixing it so my devices aren't recognisable to snoopers on a network is also somewthing which I am interested in knowing how to do.
Well, the first order of business is to find out if this is all just a bunch of smoke-and-mirrors on the part of the person harassing you, or if they have actually gained access to something. If they have access, you need to find out what and how.
I'd advise changing passwords on ALL online accounts, resetting the wifi router to factory defaults and setting it up securely, and installing Little Snitch on your Mac. (Little Snitch is difficult to configure correctly, and will warn you about all kinds of normal processes. You will learn a lot about what your Mac connects to on a daily basis, and if there's actually any hacking going on, Little Snitch should catch it and prevent it.)
I'd also advise you to take a look at my Mac Malware Guide for more information on what kind of malware threats are out there, how your Mac protects you and what you need to do to stay safe.