It's interesting, from most of your posts it reads like you're blaming the players for this. In my experience culture comes from the top down, not from the bottom up. There's been a lot written, by plenty of different people, about Milford and Boyd ruining the culture of the team, not even trying, infecting the rest of the squad and so forth.
I'd see it more as a player like Milford being a canary in the coalmine sort - when the team is no good he'll be no good, when the culture goes south then his will too. Again from my personal experience, being in a team where it all goes off the rails is infectious and turning over staff didn't help - it actually made it worse as nobody wants to take any accountability when all that means is getting moved on, so everyone focused on keeping their heads down and avoiding the blame for any failure. What changed things in the end was a new manager who actually thought about strategies to best use the staff at his disposal and held himself and others to account for what they were supposed to do without throwing anyone under the bus. I'm undoubtedly coloured in some way from my experiences but I would expect to see far better performances from Milford under a better coach.
For the reasons you state at the start of your post, I don't usually like blaming individuals as the single, or main reason for a team's poor performances. Of course, that contributes, but in isolation, it has insufficient explanatory power. I always look for the bigger picture, as do you, if I read you correctly.
As far as Milford and Boyd are concerned, the "smaller" picture is really important because it affects any big picture solutions. it's a question of them taking personal responsibility for the problems they have publicly acknowledged, and actually doing something about it, at the personal level. It's as much about them as individuals as the coaching, or the culture.
My thoughts now are that their poor performances can in large part be traced back to Bennett's departure, and the way that happened. Also, in Milford's case, I wonder if there is also something rooted in his personality, to explain his fall from grace, for example, when he refused to be mentored by Lockyer. Maybe a kind of arrogance, or maybe just insecurity. No matter, they do not seem to be supporting Seibold whatever one thinks of him and his methods. Both Milford and Boyd to me seem stuck in the past and refuse to adapt to the present. Or can't.
In any case, my issue with them is that change won't happen unless they acknowledge that they need to change. Without that, there is no chance of change happening, and change it must. We can all see that. So far, I have detected nothing from either of them that they know what their problems are. Or are prepared to even acknowledge specific challenges. They talk about being disappointed by their form, yet offer no concrete solutions. It seems to me they are in some kind of denial, which prevents change from happening. They offer only lip service. That is why I am being hard on them both. They are both well paid professionals with a special standing in this team as senior players, as leaders. They can't just withdraw and do nothing.
Because of that, their negativity remains, and proliferates, and percolates, throughout the team, affecting everyone, especially our young brigade.
I wonder, if the club brought in a psychologist (which I reckon they ought to, ASAP) whether Milford and Boyd would be honest about what bugs them, what to them, explains their stated poor performances. I wonder if in fact they would not only acknowledge they have issues but want to deal with them.
You can lead a horse to water ... and then count the skeletons around the well