Your questions seem to be about the tactical handling of a rifle company in combat and what the correct procedure was for the handling of the company headquarters's vehicles and the company's snipers. I am not an expert on Second World War infantry tactics but I have been researching Canadian war establishments for many years now. Before I put in my 2 cents there are four things I have learned that I would like to pass on.
1) The greater ones interest becomes the more one realizes that the answers are not on publicly available internet forums or books. If you're lucky enough to be living in Ottawa or near to a military museum, use the resources available. The LAC and DHH are great places to rummage through files, as are the museums. If you can't do this, then go to your local military shows and purchase some manuals etc. These also have a wealth of information. There is a cautionary note however, sometimes the official documents can be wrong too!
2) A war establishment (NOT an order of battle) is an administrative document. It does not deal with tactics or the four phases of warfare. There were 33 Canadian rifle battalions at the height of the fighting in Europe and I would bet that probably none of them matched the official war establishment at any given time. This was not due to a lack of discipline by the commander but to the situation and available resources confronting the battalion.
3) On the cover of every war establishment is a paragraph that states the written establishment is only a guide for the commanding officer, and that the commanding officer can make any changes he sees fit within the total numbers of personnel, allotted ranks and equipment provided. This meant that the CO could, if required, keep four rifle companies of 125 each or reorganize to five companies of 100 or three companies of 166. The CO's discretion was paramount and flexibility was a key word in reacting to any combat situation.
4) By the latter years of the war the Canadian Army was fighting by echelons. This was reflected in the training manuals of the time and was known as battle organization or organization for battle. This is important to remember especially when dealing with the allocation of softskin vehicles.
Now to your specific questions. Keith Matthews had some good information on infantry use vehicles, but remember two things. First, the allocation of the company's vehicles depended on what phase of warfare the company was involved in. Second, the company commander (like the battalion commander) could allocate whichever vehicle he had to whichever echelon he wanted. Generally I would think that the 15-cwt truck operated between the 'B' and 'A' echelons while the carrier operated between 'A' and 'F'. The jeep could probably do both, if required. Again, commander's discretion and flexibility. With 132 Canadian rifle companies in combat there were probably 132 SOP's.
The same can be said for the snipers. The procedure for their usage came from the company commander and depended upon the situation at hand. Their positions were transferred to battalion control for two reasons; to enhance their usage as a battalion rather than company asset, and to provide for a rank structure that allowed soldiers with a desire to get ahead to remain as snipers.
I don't want to sound pedantic but with regard to the allocation of PIAT's, remember; commander's discretion, the applicable phase of war and the situation at hand.
Now for the orderlies. The definition of an orderly in 'Infantry Training 1937' is 'a man detailed to carry messages'. This is what they did and what they were for. Remember that even by 1944 the use of radios below company level was not a fait accompli. The relevant manual, 'Infantry Training 1944 Part VIII', stated that 'the normal method of passing information back to company headquarters from a forward position is by runner (read orderly)'. Three rifle platoons in a company required three orderlies in company headquarters to pass on orders in real time.
Well, I hope this helps. Good luck in your research.