I use both seeds & powder, and also a combination of yellow & brown mustard (in both seed & powder). It is hard to find brown mustard powder here locally, but it is easily available online - Penzey's is one good source. Using some of each adds complexity, but certainly isn't necessary.
I have used a small coffee grinder to grind the seeds, but recently have been using the stick immersion grinder. I have found it difficult (time-consuming) to grind the seeds fine, so I combine them with mustard powder. I like a grainy mustard so this isn't a problem for me. I just grind a reasonable amount & call it good, adding the powder so that I get a grainy mustard in a somewhat creamy base. You can go the other route and use just mustard powder if you prefer a smooth, creamy mustard.
If you leave the mustard out at room temp (for up to several weeks), it will cure and mellow gradually. Once it's put into the fridge, the cold temp prevents further mellowing. I usually don't bother with this as I add habanero, ginger, horseradish, etc (some or all in any given batch) for additional flavor & heat - not interested in 'mellowing' it.
Mustard is so easy and forgiving. You can't hardly screw it up. It will come out differently depending on what you do, but will almost always be as good or better than commercial mustards. And, like with beer, you can make it to fit your own taste and preferences.
I've never 'fermented' mustard, but my understanding is that you add some whey and then let it sit for at least a few days, allowing a lacto-fermentation to occur. May try it with my next batch just to see how it is. Will report back when I do, though it may be a while as I just made a batch of mustard. I usually do a double batch of the recipe that I posted when I started this thread. That makes for a bunch of jars of mustard. I give away a number of jars from each batch to family & friends, and then keep the rest in my garage beer fridge till I go through them.
Anyhow, I'll post back when I get around to trying a batch with lacto. I'm guessing the whey has enough lacto on it to start the fermentation, but don't see why we couldn't pitch our own yeast or bugs. Maybe I'll try splitting a batch and using whey, some cultured lacto, some ale yeast, and also some brett in separate jars. Brett mustard, hmmmm.... I like the idea. Worth a shot. Anyway, I'll report back when I get around to trying to ferment the mustard (but again, it's certainly not necessary to do so. Mustard is easy & forgiving).
Go for whatever flavor(s) you like,