First and foremost, nice work on the diorama. It tells its story well.
In reference to the time frame, the answers to the questions of the dates the United States Marine Corps began fielding the M-16 in the numbers depicted in the scene are in your own references, Fausto. Please understand that this is an attempt at constructive criticism - take it with a grain of salt.
You cite the quote by the U.S. Marine who had written about the failures of the the M-16 on the battlefields in Vietnam. Go back to Wikipedia where the material originally came from and click through the footnotes. The letter was first made public in February of 1967 when Time magazine ran the story which later inspired the Congressional Hearings over the issues with the rifles.
Congress is pretty weird. They like to document a lot of strange things, particularly where their money is supposed to be going. A hearing is even more important. Check the published results of that hearing here:
http://www.dtic.mil/dtic/tr/fulltext/u2/a953110.pdf
It's a long document, so here's the highlights -
The fiscal allocations for the various branches of the U.S. Armed Forces placing orders for the XM-16 and the later M-16/-A1 models are broken down. The United States Marine Corps placed its first order of 91,872 M-16 rifles in fiscal year 1966. As the report states, until late 1966, the use of the M-16 rifle platform was limited to United States Army's Special Forces, Air Assault/Air Mobile, Airborne, and Ranger units. Think of Hal Moore and the 7th Cav at LZ X-Ray because it was his testimony after the Battle of Ia Drang which convinced a lot of brass to acquire the rifle.
The Marines received their first shipments from Colt in March of 1966 and the initial production was used in testing at USMC posts in the States. Congress notes that during '66, Colt was able to maintain production levels to meet contractual demands, but ammunition shortages and the fouled powder you have already pointed out limited the widespread issue of the M-16 for Marine combat units deploying to Vietnam. The rifleman you quoted received his rifle in Okinawa in late January of 1967.
If you want to stay with the 1965 date, open the manual you have in your post. My dad had given me his 1967 issue in its plastic wrapper when I was just a kid and I used it a few times in my college ROTC days to teach younger cadets how to clear a jam, why it was important to maintain a clean rifle, and to give little glimpses into the history of the weapon. It's a great resource because it has diagrams of every single variant of the M-16 the United States military had played with to that date. If memory serves, the version fielded in 1965 had a different flash suppressor/bayonet lug set-up, no forward assist assembly, and a very distinctive magazine with a stamped waffle-iron pattern on the side. Cheap aluminum construction of those mags led to feed issues detailed in the Congressional report.
Research is one of those subtle skills in modelling, Fausto. If your goal in creating such elaborate dioramas is to capture faithful details of specific moments in time, good research is everything. Be careful of the older fellows who haunt this site - they grew up before Google and Wikipedia. If you ask for a website, we'll make you read Congressional hearing reports in pennance for posting the weak nonsense with the Pegasus figures. You'll learn more that way.