If you have any kind of bonus point system, it will create alleged anomalies.
If you award points for big try hauls, then of course there will be a flip-side. In this case, draws aren't worth very much.
(In practice, the funny-shaped ball game creates so few draws that it's not a big issue. In the sensible-shaped ball game, 3-points-for-a-win makes a big difference to tactics.)
My personal preference in these round-robin systems is to have Result-between-the-two-teams as the first "Decider" - in which case Wales could not finish behind England.
Agreed.
I know you are amongst those who don't consider chess a sport, but because of the nature of the game there are some well-tried and tested means of breaking ties, which are very common in the typical weekend Swiss-paired event - a hybrid between league and knockout tournament in which typically a group of 40-odd competitors will compete over only 6 rounds. Basically, your opponent in each round will be, as far as possible, on the same score as you are.
When I began controlling tournaments over 30 years ago, the first tie-break was sum-of-progressive-scores, which gives weight to games won early on. The chances are that if your SOPS is high, you will have had to play stronger opponents than someone whose SOPS is low. The next tie-break was sum-of-opponents'-scores. The drawback of this was that in the final round two very strong players might be looking at what a couple of rabbits on board 18 were doing before they knew who had won.
Then, someone had the bright idea of taking the game between the protagonists as the first tie-break criterion and FIDE changed the rules. It generally works well, but I can remember one event where there was a tie for first place in which player A beat B, B beat C, C beat D and D beat A. It was our club championship and I was one of the 4. I think in the end we agreed to share first place.
If England do win the title as a result of a Wales-Ireland draw, that will be most unsatisfactory. This bonus point nonsense really makes it pot luck. If a good side plays Italy on a lovey dry pitch in Rome in March, they are far more likely to notch up a hatful of points than if they play them in Murrayfield on a wet February afternoon when there's little chance to show off ball-handling skills.