The RSL Guidelines differ in referee "instruction" at certain areas, and allow more room for interpretation by the referee with regards to materiality of offenses.
A. The Tackle
Defenders, want to slow down ball availability. In the RSL we decided that if we have to instruct the tackler to move away, that they have already slowed down ball possession and should be liable to PK. If attacking players arrive at a tackle, and do not make contact with opposition players, but 2 or 3 of them huddle over the ball, no ruck is formed and there are no offside lines, except for the existing tackle gate. If a defender counter rucks, a ruck is formed. We work with the referees to get to the tackle early, ensure the tackler has released the tackled player and the tackled player exercised their options. Only if a ruck forms, AND the ball is secured, to then ensure the fringes are onside, and then to move out to manage the backlines. A big issue is that referees do not get in to manage the tackle, but rather remain some distance away, and assume a ruck forms on most occasions.
B. The Gate
You question the "square shoulders". It was suggested by a coach, as at times the gate is wide, and at times, narrow. It is a part of the game that is refereed inconsistently, but that referees would give preference to players in the "square shoulders" entry, as it would visually appear that the player is entering correctly. Your diagram is correct that all 4 players are legally entering through the gate.
C. Arriving Players
The answer is that the guidelines are applicable to all rugby, and include iRB Directives, and serve as clarification of Law and application. An example is that at international level, as seen in some (NOT ALL) Six Nations matches, referees instruct the tackler to move away, which they then do, which has already slowed down ball recycling, and allows the defense that extra time to get into position, and on almost all occasions when this is consistent throughout, leads to a low scoring game. Quick ball availability = dynamic rugby.
The player in the illustration that you refer to as obstruction, is not. The tackle area is dynamic, and often many things happen very quickly, so the player does not necesarily remain static in this position for very long. We will allow players to take that position over or very near the ball. It is allowed internationally. If the player gets far beyond the ball it would be obstruction. It is a subjective judgement by the referee.
Your view on the "diving over" is correct. Here is a video from the RSL 2009 which clarifies what is allowed and what is not. It is an 18 minute video with 40 clips. Probably best to view it more than once. (http://richardevery.com/Tackle.wmv)