Geddy

So you changed the actual content in the custom notification? If you go to the Configuration -> Emails/Notifications -> Customizations tab then click on the area for "Customized Content" it should open up the actual message that it is sending. Make sure you fixed the typo in there. Either the notification didn't change if you had it specific to a device rather than a node variable.
It appears that your typo is in the email account setup itself.  It's showing a from "Spriniler System". That is more than likely an email system issue. 
What is shown in your "SMTP Settings" area? 
 

UD Mobile is for ISY 5.0.16 and above.  (Currently for ISY994 devices without z-wave I would suggest 5.3.4)
Learn more about UD Mobile in the UD Wiki.
I have about as many devices as you do, but they're all 2015 and newer (roughly). Thankfully I haven't had the same fail rate that you're experiencing so perhaps your units are much older or you have power supply issues. Either way, you might want to see if you've got a home electrical problem rather than an Insteon problem just to be sure something isn't frying those devices. You wouldn't want to switch to a different type of device and still have the same fail rate.
Since you're only Insteon (and few x10) then I'd say your upgrade to 5.3.4 should be mostly smooth. The only major issue going from 4.9 to 5.3.4 is if you have programs that use adjust scene. This is because of the change of how scenes are handled in 5.x.
The key (tip/trick) is to copy all programs to a text editor so you can refer back them after the upgrade and adjust accordingly. Never expect to adjust (or recreate) programs from memory. 
In the program tab of the admin console right click the root "My Programs" folder (of if you changed the name whatever the root folder is). Then select Copy Folder to Clipboard (note...last option in screenshot below). Now, paste that to a text editor (in Windows I suggest Notepad - just something that will NOT auto format any text). Make sure you do not stop at the option to "export to clipboard". That will give you an xml file and be more difficult to read. The text pasted from the "Copy Folder to Clipboard" option is clean and clear to follow for reference.

What you will get in Notepad will start like this: (very easy to read programs this way)
=================================================================================== My Programs - [ID 0001][Parent 0000] Folder Conditions for 'My Programs' If - No Conditions - (To add one, press 'Schedule' or 'Condition') Then Allow the programs in this folder to run. Another note to keep up with...make note of any disabled programs you might have. When I upgrade from 4.9 to 5.3.x all my disabled programs became enabled and I had to turn them off again. Some around here will mark disabled programs by name or put them in a folder called disabled.  Disabled programs will show up in your text file with [Not Enabled] clearly shown.
Now is also a good time to go through and possibly rename programs so that ones that run processes in the same room or rely on other programs might be close together in the name list for ease of reference.
It's been almost a year since I upgraded, but still remember that it took me less than an hour to make the upgrade and confirm/adjust programs. Then, over the following weekend if I noticed programs not doing what I expected just spent a little time fixing those up. There weren't many, but there were a few minor tweaks to make.
 
Once up and running with 5.3.4 then you can begin using UD Mobile. It's really becoming a great addition to the ISY world and becoming quite powerful with options. 
Final tip - BACKUP! BACKUP! BACKUP! Make sure you backup your current setup before you attempt the upgrade. Since it's a manual upgrade that's a step in the process, but it's good to run one before attempting the manual upgrade too.
Then, when you get to step 7 in the process just do the first part:
This will download/add the ISY Launcher process for accessing Admin Console. (If you're on Windows this is a simple process...on Mac it's a mixed bag) Run the "start.jnlp" once and it will install the ISY Launcher icon to your desktop. Now, always use this to access the admin console. This will usually open the "ISY Finder" window and should automatically list the local ISY (as long as it is found on the local network). 

 
If after you go through the upgrade process and have issues jump back here and let us know what issues you're having and somebody might be able to help. Otherwise, open a support ticket with Universal Devices (https://www.universal-devices.com/my-tickets/) for individual support for your issue.
Good luck! 

@Bob Amott welcome to the forums. Sorry you're having difficulty. Sadly, I do not use macOS so can't help very much other than try to point you in some generic directions. Do you happen to have a Windows computer you could try this on? There have been some mac users that sail through without issues and others that something in the system just blocks everything they try. 
But for starters make sure you check out the wiki for Mac user troubleshooting:
https://wiki.universal-devices.com/index.php?title=Main_Page#MAC_OSX
and here:   https://wiki.universal-devices.com/index.php?title=Main_Page#MAC_Users
 
Are you going to continue to use the 994i or are you attempting to setup ISY on Polisy (IoP)? Indeed if you're going to migrate to IoP then you will need to update the 994i to 5.3.4 before you can then attempt to migrate to IoP. 
 
The firmware for the 994i is looking for a .zip file. I'm not sure if the macOS is saving the .zip file to what you're saying is .img file type, but this is the first issue. The file downloaded should remain a .zip (compressed) file. Do not unzip/decompress it. 
 
Finally, and as much as I hate to say this...probably best to open a ticket and get Universal Support to walk you through specific steps for macOS for full upgrade. Upgrading from 4.x to 5.x is very complicated and if something isn't right there's a lot that can go wrong trying to help through forum posts. So hopefully support is better able to handle live help and support on macOS.
 
 

I agree with @DennisC, you should upgrade to 5.3.4. It's the most recent supported firmware for ISY994i. While there might be very few changes from 5.3.3 to 5.3.4 it's enough that it would be the first step in further troubleshooting any other issues you have.
As do your exact experience - your brother has the "Launcher" which loads the finder and allows the computer to download the UI to properly match the firmware with every launch. (Thus, "Launcher")
You downloaded the admin console for the direct version you installed. It would be just as though you downloaded admin.jnlp from the ISY994i directly. 
These two routes to access the Admin Console act differently, but they are in fact the same once you access the ISY994i. 
Since the Launcher was introduced in 2018 that is the most often suggested and the recommended process for accessing the ISY994i admin console. It was designed with 5.x in mind as beta builds were being released so users would always have firmware/user interface matching. 
So nothing is "wrong" in your setup.  You're just taking different routes to get to the same destination. There have been a lot of reports of the font/display to be different as we moved into 5.x. You can change those settings, but odds are they won't stay for long as they tend to be dropped and need to be reset.
But, be sure to go ahead and upgrade to 5.3.4 (since you're so close it should be very quick and not require any additional effort, but be sure to follow the steps as outlined on the release post.