It would be kind of silly if you couldn't group up devices like that. If you think about it, modern networks work very much like this. Take a modern-day setup. We have companies with servers that all slave a number of computers to themselves. Those computers, in turn, will slave other devices (such as flash drives, mice, keyboards, monitors, ect.)
Your rational is correct. You can, indeed, create groups of devices, slave them to a commlink, and then have the decker slave the commlinks to himself. This creates a mobile PAN (personal area network) mesh that the decker can more easily police and protect, since he's the first thing an enemy hacker has to get through. There are, of course, problems with this setup.
The biggest flaw to this setup is what happens when someone DOES make it through your detection and firewall unchecked? The answer? They now have EVERY piece of hardware your entire team owns at their fingertips. And until you can get them out of your node, they can choose to screw with any of the devices. Normally, if they want to hack the street sam, they'd need to be within range and hack their commlink separately. It's easier, but that's multiple points of failure, since the enemy hacker can't get to anyone else on your team through the sam. With you protecting the group, you're more likely to catch any intrusion, but if you fail, the enemy hacker now has three marks on your entire team and hilarity can ensue (read: bad things for your team).
Generally, you want to protect all of your party members and slave all of their devices to your deck. Period. There is no reason NOT to do this, honestly. The chances that you'll miss a hacker in your system are pretty low anyways, and even if you do miss them, rebooting your deck will sever the connection and erase the hacker's marks on your deck, forcing the enemy hacker to have to start again on someone else's device.
Supposing that the "started turning into the creatures" part you quoted is from p.50, I'd like to draw your attention the specific mentioning of Goblinization in the introductory part of the book:
Then, in 2021 (...) That’s when Goblinization struck. And it was not
pretty. Where UGE had created interesting-looking newborns,
Goblinization struck people of all ages. The most noticeable symptom
was blinding, mind-numbing agony that came in waves. This lasted
twelve to seventy-two hours while the victims changed shape, grew
tusks and/or sprouted horns, and maybe quadrupled their body mass.
Which is how the orks and trolls came back.
Shadowrun 5th edition core, p. 20
So I think yes, Goblinization is still there as it used to be in earlier editions which you can, in my opinion, consider semi-canon: as long as something is not mentioned in SR5 or is not invalidated by what SR5 says, it's canon, though you'd of course have to keep an eye out for changes presented in later and upcoming publications as well, not just the Core... if you wanted canon.
As for your backstory, let me give you a slightly elusive answer (besides noting that as far as I can remember yes, people can undergo the change later, though that's quite rare these days, and yes, you can have a sibling from a different metatype) : Ask your GM. Seriously. There are always anomalies (people born with two heads and so on), there are rare and unique stories (that make them even more interesting), and you're playing a PC whose story is indeed supposed to be rare and very interesting.
Best Answer
One option is to re-purpose buildings already known to you. Your grandparents' house can become the floor plan of a safehouse. Your old job can be the floor plan of an office they're breaking into.
You can play around with this -- for example, in the real world, homes get turned into small offices; your parents' place can now be a dentist's. The ground floor of best friend's house can be a tiny apartment the players have to search.
This will help with memory issues (like where to fit in the bathrooms) and give you some variety: but you don't need endless variety. The dentist's office this week can be a realtor's in two weeks, it's not that big an issue if the walls are the same -- the contents of the building, where you place guards or traps or etc are what the players will be concerned with.
You can also do some sketches in advance. Make a floor plan folder full of things you think you might need in future or wished you had in the past. Do a safe house. Do an office, etc. Then practice with those a bit -- take one, and design it as one thing, then do it again as another. This has two purposes -- one, getting you used to designing the floor plans in the first place, and two, if you like them, you have at least a small stockpile you can refer to at a moment's notice.