wrote: How far will Sims walk? Can you provide me an estimated distance and how to gauge it within the game?
Do the trucks use the service roads in favour of the other types? That is helpful to know because seeing cars queued behind a truck collecting garbage is a painful sight 
If I built a train station in the Residential area and a second in the Industrial, what will happen to the commuters then? Or do they just carry on and out of the City?
Similarly to a previous question - how far will tourists walk? I read or watched somewhere that having a Municipal Bus Terminal (MBT) placed at one side of the casinos and a second MBT placed at the other side will make the tourists get off from one and walk passed all the casinos (visiting them to spend money as required) and get on at the other to leave?
Also, if the MBT brings in Sims from the Region and you've advised not placing bus stops for residents...how many bus shelters at the MBT should be placed? None?
I am slightly unsure about how and where to place bus stops...so I shall explain how I have placed mine and receive your feedback about whether this is correct. So, on one stretch of road where I have 8 high density Residential buildings I have placed 4 bus stops. One stop between two of the residential buildings.
On another stretch of road, away from this, I have 10 high density Commercial buildings and have placed one bus stop between two of these, totalling 5 stops.
On another stretch of road, away from both, I have placed 1 bus stop outside a high density Industrial building.
Which of these will work, and which won't? Which of these will cause passengers to jump between stops, one after another.
How can you work out if Student agents or Worker / Shopper agents have gone missing?
I do appreciate all of the help you are offering and apologise that each answer produces more questions!
Yes - the garbage trucks will favor the service roads. You can use oil rig service roads to go from the oil drills directly to the trade depots, too. I love that!
According to "sources" (online guides and stuff) Sims will walk 400 meters. I have no idea how far that is in the game lol. I think the included screenshots from my previous answer is the best I can do on showing you what I do. When I have roads set to about half-way across the map, putting a block of blue in the center with green on each side cuts the traffic down to manageable, and as far as I can tell, the Sims will walk from the far corners on either end to the center. If residents complain that there's not enough shopping, I'll plop another commercial building's worth to see if they find it.
Train running from one end to the other to get your workers there - probably will work. I don't use trains much, IIRC, they're a little bugged but I can't really tell you how. It's certainly worth exploring yourself to see how you like it. I would not put anything other than a station on either end. Another option would be to have a main thoroughfare with a streetcar avenue that runs to each of the major zoned areas.
The way your bus stops are set up now, I would guess that all of them will cause issues. Personally, I prefer shuttle bus stops; they directly take the workers from Point A to Point B, with no regional traffic. Try replacing your 4 Residential area mass transit stops with 1 or 2 shuttle buses with the max of 3 buses. Then replace the Industrial and Commercial mass transit stops with one each. Turn off the MBT and run a couple of Sim-days. If the traffic still needs help, try plopping Park-n-Rides in the same manner and watch your traffic for a couple more days. I think you'll be amazed by effect, especially if you move to a mixed R-C to encourage walking for the shoppers.
To check for missing students: pause the game at exactly 5:59 am. Go to your Education Data Map view and look at the bars (all of the students should be at home and "uneducated" at 5:59 am). A quick look at the bars should alert you to any that are not the same size as the surrounding residences; then you can use the Extended Data Mod to see how many students are in the home. For instance, LW MD should have 15 students. If it has any other number - 7/15, 0/15 - you have missing students. Since you have a university, you may have a greater chance of "losing" students to the region (they'll go to the university, and the agent pathing forgets that they originated in the city). Really, the quickest way to remedy it is to demolish the building and let it rebuild to reset the students. The official "day" starts at 6 am; if you suspect that a building has a problem ("no work" when there's work right next door, for instance), un-pause and watch the numbers. Not all of the workers or shoppers will leave right at 6 am; some could be night workers or shoppers, but they all should leave either at some time and come back at some time. If the number doesn't move for a full Sim-day, demolish the building to reset.
wrote: I do appreciate all of the help you are offering and apologise that each answer produces more questions!
I'm just happy that I haven't overloaded you with more than you wanted to know! More than happy to help - your questions sound a lot like the ones I had when I first started. I really hope this helps - but please, play around with stuff and enjoy 
Would love to hear updates on your city. Happy Building!