The practice version of coils or any general runs that might come later in the game does seem like a good idea, my friend just started and I know he will have a hard time finding parties at 50 because of those who demand excellence mixed with those who would help but can't due to their progression. I did think about practice modes and there are in a sense some already out there but I think it would be more useful for them to extend on the things that "teach" us.
Guildhests teach us party basics to boss specifics to a certain extend, I think it would be nice for them to put more difficult ones in that teach things from Binding Coil and beyond. This would give people a reason to do the guildhests again, learn about fights without having to take in too much time.
Dungeons already give us some practice, such as the Lost city mob using it's own Floral Trap / Devour / Spit method. Maybe expand on that, but make them add a few more steps, since these fights have such high combinations, merging together two mechanics to get people use to it would make dungeons more of a challenge / more fun for the expert ones and getting their feet wet with the new comers.
I do agree that there should be some way for those who complete their turns to come back and help out, rather than make a whole new practice mode taking the above methods and taking away progression lockouts but adding in loot lockouts all together would accomplish the same things.
I also wonder about how often the practice runs would be used, even with myth and soldiery being used it's still going to get to a point where it's hard to get 8 people to enter, while if you use the three above methods you only need 4 people for the hests and dungeons, the hests will eventually follow the same thing as a practice mode and dwindle down in usefulness but the dungeons will always remain. Thought I guess Yoshi & Co. could always open up the option of adding a very small amount of Elite Marks to those who finish these much higher practices from GuildHests.
So, while I agree on the practice method, I've been down that road and the problems I came in to eventually had me make the above solution that requires both game balance in a sense and gives people the ability to practice. eventually people will want to just practice on T7 - T8 leaving people like your friend and mine again in the dust or new people in general, this way they can use two types of learning curves (Guildhests and Dungeons) that don't require a full 8 and then take it a "step up" to the actual Turn 6, but give these people who want to help a chance to come back down and help out just with no rewards, or a gimped version of soldiery / myths for giggles. Let me state that I did rate you up and I do agree with the practice method in a sense, just after thinking it over myself I came to the above conclusion.
And though removing the lockout would bring in people selling them, you're not going to stop the selling..you can delay it or diminish it for a time being but you can never get rid of it. This is something that needs to be considered. On top of that the whole point would be so the ones who clear whether they're elite or not go back down and help. Even if you give these abilities to go back and help in a practice mode FC's will take their teams into their turns to practice their strategies. So our best bet is to place things that are already on the road map of the game to help the new players out so when the time comes for T6 they can go "Oh this is just like.."