Hey there Mr. Sarsaparilla, I'll hazard an attempt to answer your questions.
1. Lets assume that we have a device,say a sentry gun in a Renraku black city slaved to the local host. Where does it show in the matrix? Near the hacker (Assuming that the hacker is near the sentry gun) or is it inside the host and therefore invisible to the hacker? Other possibility would be that you can see the icon of the gun but a "glowing forcefield" is transmitted around it from the nearby moon of Endor..Sorry Renraku blacksite host.
The sentry gun is almost 100% likely to be on the Renraku grid and host. This depends on the architecture that your GM decides to use for the sentry gun. Most likely he/she will want to put the gun on the Renraku grid and host to take full advantage of the Matrix security that they have to offer.
The blacksite host would be running silent, but still connected to the overall Renraku grid. It's also likely that the blacksite is hardlined to the grid, making it impossible to access from outside the site. So assuming the blacksite is running a host (it probably is) and the host is wireless (which is probably is), your hacker will need to get into the building or find a way to tap the hardline that the blacksite is running to the rest of the Matrix.
You can access any grid from wherever you are, but you suffer a penalty if you're not on the same grid as your target. So you'd "hop" to the Renraku grid to find the host. Once you've gained access to the host (either by hacking it or directly tapping the line), then you have to find the sentry gun's node. Once you've got the node in sight, you can put marks on it to spoof commands, or hack it to try to take full control, etc.
2. Next stop, the problem of the nature of hosts. In SR5, it is stated both in the matrix glossary and in the description (sorry, i haven't got the PDF here with so i can't make direct quotes) of hosts that hosts are non-localized matrix entities. The chapter's intruction would support that statement by saying that for example the hosts of the big ten can be always seen hovering above. So how do you measure distance to a host? Furthermore if hosts are non-localized why would the big ten create separate hosts for example blacksites? It would then make sense to actually have everything on that one big host above the sky and put the hundreds of spiders and IC in their disposal running inside the one host and make that host virtually impregnable. This problem is made even more obscure by the statement that when you're inside a host the distance to any device slaved to it doesn't matter.
I was trying to think of a good analogy to help explain the nature of hosts, but honestly the tech doesn't exist yet to give a good analogy. Picture this instead: the hosts are all connected via a grid. The grid helps to minimize signal attenuation and packet loss, but it's not perfect because distance still applies. This is why the Big Ten don't run a single host that manages hundreds of thousands of nodes. On top of that, the hosts simply wouldn't have the power to run all of those nodes at once.
However, having all the hosts visible all at once is a huge mistake, right? Not when you consider that a host can run silent, making it invisible to casual observers. Consider also that the point at which a user jacks in to full VR is the location that is used when determining matrix perception. Most hosts, nodes, etc. are difficult to perceive at distances further than 100m away. So a potential hacker has to be closer than 100m away from the host, perceive the host, hop a grid, and possibly also tap the data lines in order to hack a host. It's a pretty secure system, really.
I get that you can measure distance to local host quite easily(just measure your distance to the closest stuffer shack etc.) but what i don't get that is, if hosts are non-localized and noise isn't an issue inside a host, why would anything owned by the big ten be outside their main hosts and guarded by a veritable army of spiders and black IC. If this is the correct reading, I have to admit running against a Renraku blacksite went from crazy-dangerous to mission impossible on steroids. Basically that reading would mean anything connected to the big ten would be basically out of reach, even the local Renraku office gopher's Honda Gopher...
As stated in my previous answer, the hosts simply wouldn't have the power to do what you're describing. On top of that, there's the fact that doing what you describe is a huge mistake from a security perspective. That would mean that Joe Schmo at the Stuffer Shack in Des Moines can access (via hacking, naturally) the Aztechnology data in Cali, Columbia.
