5 meter or a bit over 16 feet, that's pretty far.
If you have an external PHONO Pre-Amp, that can, in a sense, act as a relay. Just to illustrate, you could have 6 feet of wire from the turntable to the Phono Pre-amp, and another 6 feet from the Phono Pre-Amp to the Stereo. That is not 12 feet, functionally that is only 6 feet, as I said, the pre-amp acts as a relay. (Those numbers are strictly for illustration).
Whether you can do that with two 8 foot (2.5m) cable or not is unclear.
I too have my turntable a bit far from my amp, but in my case, I've simply patched two 1 meter cables together. It seems to work fine.
The problem is in trying to determine when the length of low level cable gets long enough that the resistance, capacitance, and inductance start to effect the sound quality. Also, the longer the cable, the more it acts as an antenna and is likely to pick up noise from the air. Most likely electrical power line noise (50hz).
The first thing you have to wonder is if you can actually find 8 foot (2.5m) low level signal cables. Then I would recommend getting high quality cables, though, one doesn't have to go crazy on the price. Just not cheap cables.
Most of the RCA-RCA cables I find are in the 1 meter to 1.5 meter range. However, you can have cables custom made in any even meter length -
QED Performance Audio 2 Custom-Made Stereo Phono / RCA Cable - By Brand - AudioVisual Online - Home Cinema and Hifi Specialists
In your case probably a 2 meter from the Turntable to the Pre-amp and 3 meters from the Pre-amp to the stereo.
It is possible to get RCA-RCA cable as long as 3 meters (9.8ft), but short of custom made, that is about the longest I can fine on a quick check -
QED Qunex 1 Stereo Phono / RCA Cable 3m - By Brand - AudioVisual Online - Home Cinema and Hifi Specialists
Fisual Super Pearl Stereo Phono / RCA Cable 3m - By Brand - AudioVisual Online - Home Cinema and Hifi Specialists
The underlying question isn't - can it be done, you can do anything. The true underlying question is - how well will it work. That is a bit more unpredictable. I suspect, if you have good quality cable, and the quality of the power line voltage in your house is good, it will likely work acceptable well.
Though that also depends on the quality of your equipment. With a typical consumer stereo (or audio system) a slight compromise in sound quality is not so noticeable, but with a very high end system,
everything is noticeable.
If you have an external Phono Pre-Amp, as you imply, what comes out of it is new fresh signal, so it can act to boost and relay the signal to the amp. If you place is set up so the Phone Pre-Amp can be near the midway point, then you really only have 8 feet (2.5m) of cable to worry about. I don't think that will be a problem.
If you have to split the lines, say rather than two 2.5m cables, you have to use a 2m and a 3m, put the shortest cable on the turntable. The signal from the turntable is extremely low and very susceptible to noise, so it should have the shortest and highest quality cable.
I would give some serious thought to this plan though. If you can find an alternative to the turntable being so far away, the shorter distance would be preferable.
As to wall mounting a shelf for the turntable, like anything, this has advantages and disadvantages. When I was young, I lived is what is best described as a "Party House". I had my turntable on a floor stand (equipment stand), my roommate has his mounted on the wall. To keep my turntable from skipping people had to walk delicately. Not tip-toe, just mindful. However, relative to my roommate's turntable, people had to be careful not to SLAM the front door. When the door was slammed, that shock wave was transferred through the walls and made the turntable skip. Though bear in mind, this was a wood frame house. This reaction is far less likely in a stone, mortar, brick house.
As to the wood frame studs with plaster over lath, that's not as bad as you think. I bought a Stud Finder, but not a fancy electronic one, this is a simple magnet floating in a plastic frame. To fasten the Lathe to the Studs, they use nails. That means at every vertical stud, there is a long vertical row of nails. Slide the plastic frame along the wall, and when the magnet finds a row of nails it points at the metal. Re-check above and below to make sure you are actually on a stud, and not a random bit of metal in the plaster, and you've found a Stud. If you fasten the wall shelf to the studs, you can be sure it will support substantial weight.
This little magnetic stud finder was only about £2. I mean it is just a plastic frame with a magnet on a pivot, so nothing fancy.
STANLEY Hand Tools : 47-400 - Magnetic Stud Finder
Stanley 47-400 Small Magnetic Stud Finder with Marking Notch | eBay
There are shelves just for turntables, and I think they are intentionally made to fit right on the stud frame centers, which I believe is about 430mm center to center.
Superfi - APOLLO WT1 SE HIFI WALL SHELF
Pro-Ject Wallmount IT 1 Turntable Shelf
Pro-Ject Wallmount IT 2 Turntable Shelf
Rega Turntable Wall Shelf - (Black) | eBay
Custom Design Wall Mounted Turntable Support - Black Frame, Clear Glass Shelf | eBay
I'm currently using 2 meters on my turntable, and that seems workable. So, depending on the circumstances, if you put 2 meter between the turntable and the Phono Pre-Amp and 3 meters after the Phono Pre-Amp, I suspect, that would be workable.
However, I'm not sure about a full 5m run of a single cable. That is likely to work, I just can't predict to what degree it will compromise the signal, perhaps none, perhaps ... more than none.
If you do use a full 5m run, then it is best if that run is after the Phono Pre-Amp, where the signal is stronger. You would not want 5 meters connected directly to the turntable. This direct TT signal is about 100 times smaller than the Line Level of the Pre-Amp out.
I'm sure I have not truly answered your question, but perhaps I have added some helpful perspective.
Steve/bluewizard