^^ Once again, it doesn't matter whether or not Nate and Brandon are retiring or not. It's fairly likely that they aren't coming back to compete next season. But competing or not, doesn't have direct bearing on them assisting in coaching younger teams. I bet they've been helping out all along as veteran pairs guys. And now, in Todd's absence, as he recuperates, these guys are obviously stepping up even more to help out.
When Nate and Brandon were competing, they even supported each other. And if they came back, helping Isabelle/ Ryan as a young team who are still learning, is not a conflict. As I said earlier, K/F and C/J were direct rivals at Irvine, but they supported each other, and they were/ are good friends off-the-ice.
From personal experience (though not in figure skating), supporting each other or being a mentor to less experienced athletes is not even remotely in the same ballpark as being a coach. Apart from a difference in monetary aspects (as not all coaching positions are paid), there's a big difference in time and energy invested.
It's easy to give tips when athletes come to you with questions, and if your training times overlap, it's also often easy enough to catch certain issues and give advice (as well as just in general being used by your coaches as a demonstration tool for others to model their technique, etc. after), but actually coaching can't just be done as an afterthought or simultaneously with your own training. Or, I mean it can, but not really well.
Now, Nate and Brandon would be part of a much bigger coaching group, so it would be probably possible, if still difficult and stressful, to coach an elite team and at the same time keep up with an elite-level training plan, the question is more so would they want to add to their already hectic schedules? I could see Nate doing it for money, as he doesn't have as many opportunities to earn money in shows for example, and his season is not as packed as Brandon's would be, but Brandon makes good money with shows and competition results, and if his next season looked similar to this season, he would not have a lot of spare time to coach a team like Isabelle and Ryan.
And as for whether or not Isabelle and Ryan are direct competitors to Knierim/Frazier and McBeath/Bartholomay, that depends on what you consider as direct competitors. They'd likely all compete against each other at Sr Nats, and while K/F's scores are far removed from what Martins/Bedard are likely to achieve, McBeath/Bartholomay's are not. Ma/Be scored 152.33 in juniors, with easier content than they seem to be potentially capable of now (obviously, potential is one thing, the other is managing to deliver when it counts) and one less lift (around 6-8 points) less in the Free. Mc/Ba scored 172.74 at the highest last season, but their lowest score was just 147.29. Nate's back has unfortunately been a significant and recurring issue for them last season, and while it's never nice for someone to have to retire because of injuries, I do hope he does what's best for him and his health long-term, whether or not that ends in his retirement.