Technically speaking the ground block is susposed to go at the point of penitration, meaning it goes right before the cable enters the home.
To mount the ground block to brick, simply go to the hardware store and get some of those plastic anchors and pre-drill a hole with a masonary bit.
Most of the time on Pre-wired homes, we'll usually install the ground block in the basement ceiling after the cable has already entered the home and use it to make the final connections to the customers wiring.
Preferably, the less connections you can make outside, the less chance of a service call you will have in the future from water getting inside the connections.
You should really have your ground block outside, but most installers will mount it inside. In addition if your using a multi-switch, you can usually ground off that.
The ground wire will not protect agenst lightning, it will simply help static electricity find a safe way to ground before it shorts out your receiver or LNB.
The easiest way to tell if there are splitters in the lines is to count the number of wall plates and then count the number of cables in the basement. Usually they are all home run, sometimes its common to see all the 1st floor wall plates home ran and then a single cable going to the attic with a splitter.
There are some toners which will detect splitters, but I find it faster to simply to do trial and error. Get your good satellite feed to the central point and start hooking stuff up to see if it works.
When using existing cable, its very important to check the signal strenghs on all your transponders before leaving. If the signal is un-usually low on a few transponers, it could mean a hidden splitter in the wall or a bad piece of cable which will cause a service issue in the future.
Now to be honest with you, there has been many times where I have looked at the customers existing wiring for cable or another satellite system and it was easier to just rip it all down and run new cable than it was to spend a few hours troubleshooting. You'll be surprised how fast it is to run new cable when you can already use the holes that are already there.
The only time you should use existing wiring is it was in the walls and the customer has wall plates to all the rooms. If it was run through the floor and along the outside of the house, you need to determine the condition of the cable, and only use it if its nice clean runs. If you see any splitters or anything, its better to pull it out and replace it than risk having to go back later on a service call.
Remember, your being paid to install all new cable from scratch. Just because the customer already has cable installed doesn't automatically mean you have to use it.