The problem is that it is full of subtleties.
This is why it is not just one steward (referee). It is also why guidance is provided from a professional racing driver – even though any final decision is not solely his or hers. For comparison, one might ask of football, "when is bodily contact a foul or not a foul?"; and even within this (or almost any) sport, there are likely to be differences of opinion between professional referees who know the rules inside out, never mind the onlooker…
Some competitions which require an arbiter are easier to manage than others. The rules of chess, for instance, have 'absolute' rules* about how pieces may be moved – which all-but eliminates the need for judgement over how a player makes a move – but there are subtleties which apply over such matters as claims for draws, time-keeping and general conduct. No doubt there are others which as a layman I am ignorant of and would be confused over if I were to witness them.
One of the things about motor racing is its subtlety (not to suggest that other sports are less s0). The over-riding 'rule of engagement' applicable to all overtaking is safety. Safety within an inherently risky sport? Hmm… maybe now it doesn't sound quite so simple. For instance, let us try defining safety such that:
- it may be written down perfectly without clauses and sub-clauses
- it is always interpreted correctly as intended
- it eliminates potential for any and all 'loopholes' thus doing away with any and all argument
- it is judged consistently by all parties.
One might even add a fifth element: eliminate the possibility of misrepresentation by lay-persons such as journalists and other propagandists, whether they might do so deliberately or through ignorance. Thus what may look or be correct and acceptable to the general public – and deemed as such one weekend – may look or be incorrect / not acceptable / not deemed as such at the next.
In an attempt to clear up doubt, I will address the basic questions raised by Smithers.
- The 'racing line' is the normal line – i.e that accepted by experts (top level competitors) as most likely to reduce lap time – and that which is taken through a corner/turn/curve in the complete absence of others. Clearly, the presence of one or more other vehicles impinges such a norm. However, this does not eliminate it from any discussion of overtaking: it remains a significant factor but it is just one of many.
- 'Entitlement' in the presence of others is a misnomer. There is no such thing any more than there is a 'right of way' for a driver on the road**!!
- 'Crowding'(?). A racer should not drive in a manner likely to endanger a fellow competitor. Endanger? – See 'safety'…
Oddly enough, within motor-racing, what might be deemed crowding by an onlooker may not necessarily be correct. This is because it can appear that one or other driver had no alternative when in fact it might be judged that there was one available! It depends on a number of factors, not least that whomever is on the receiving end has an available alternative without the need to go outside track limits. Two examples: 1/Hamilton v Rosberg, Texas. Hamilton forced Rosberg into a choice because Rosberg could see what was happening: i.e. he could go off track in an effort to continue the fight after regaining the track; he could run into Hamilton (which would have been an avoidable collision he'd have been held responsible for); or he could back off – which sensibly, he did. 2/ Räikkönen v Bottas, Mexico. Bottas was sufficiently alongside and knew that Räikkönen had seen him (since the latter had already accepted the need to deviate from the 'racing line') before the collision. This demonstrated that Räikkönen knew he could not make the apex without causing a collision because he knew Bottas was already there!
I hope it will be seen this post could go on and on and on… … … …

*P.S. My limited understanding of chess is that even the rules of piece movement have clauses!
** For the motorist: priority, yes; right of way; no. Even faced with a green light, a motor vehicle does not have 'right of way'! It is a shame this misinterpretation has taken hold over many generations; truly, it is incorrect. Rights of way refer to a legal entitlement to use footpaths, bridle-ways, so-called 'green-lanes' etc., etc. It might also be said that any legally entitled motorist has a right (the 'right of way') to use public roads; however, when interacting with others, there is no right of way as such, any more than there is for a shopper with a trolley in a supermarket!
I understand the above - my point was that since the introduction of these rules and guidelines (and how they have evolved with differing stewards) they have over complicated the situation. Ourselves in yesteryear would laugh at what we now deem as unacceptable.
To play devils advocate you use the crowding rule to absolve Lewis for the turn 1 Austin. For Nico to crowd Lewis he would have had to have pinched Lewis at the apex (which is why people are viewing Kimi to be at fault). Once Lewis had left/past the apex because he hadn't approached the corner on the "racing line", would the crowding have become an action of Lewis rather than Nico? Alternatively Nico gave him a cars width in entry to not be given a cars width on exit.
The use of the word avoidable is also debatable - Everything is avoidable subject to Driver A yielding to Driver B, but where does that yield start? Does it start when the lights go out, when the 2nd clutch engages, when Lewis gets along side, when Nico affords him a cars width, when Nico roles of the brake to inch ahead in the corner, when Lewis has compromised his entry and braking point under steers off the racing line - or when Nico makes the decision to avoid crashing - which you have suggested would have been his fault.
Maybe Kimi shouldn't have afforded Bottas a cars width on the outside of the left, and run him off the track so he wouldn't have the "inside line" for the next corner - which was not the "racing line". A car pinching another car on the blindside on an Apex "should know the other car is there" but a car on the outside that you can clearly see is OK to run out of road?
Nico v Riccardo (Germany?). Riccardo out brakes himself into turn 1, Nico allows space by holding the outside/racing line (is that a yield?) and Riccardo goes off the track. He is now off the track or as he rejoins on the "outside" of turn 1 and behind - who yields - and who has to afford the other a cars width?
Every situation is different, but every situation is the same. It's not that I disagree with decisions per say, but that drivers are interpreting what they class as avoidable differently. Charlie wanted to encourage overtaking, but in doing so the definition of when or if to yield has become even more clouded. It has effectively just moved the accident further around the corner.
For the record (and I've said this in previous threads) I blame Nico for putting himself in the position to be "run off" track by not conceeding the corner and hanging onto the outside line. Anybody who has raced will tell you that is suicidal. But it seems the rules now allow a driver to force another driver into avoiding this action.