A few old threads talk about the disappointment from the Mini Brews not sealing, thereby making airlocks not bubble. Search them for details from the pros...
My Mini-Brew does not have the gasket and I cannot purchase one for my particular older model, so I tried to make my own and make the lid fit TIGHT...no luck. I just keep a threaded plug in the lid's port, and use that to peek inside without disturbing the lid, and just trust that the CO2 blanket and the snug lid will handle things like the manufacturer says. No problems so far.
Yes on the bottom valve/no usual need for secondary. I dump any trub remaining from the boil a few hours after pitching, then a quick dump once fermentation begins to settle down, and then maybe one more the day before bottling.
Since brew day, I factored in the volume below the bottling valve (my 15G model has .75G below the port) and planned for an extra .75G to begin with. Even if I sampled a lot or had a mishap in transfer, I've still got that cushion and could bottle from the bottom valve, if need be.
Sorry, don't know about the dry hops. That bottom port is decently sized, and I put on a matching 1/2" ball valve, but I bet I could make it clog. I would consider a loose bag if I ever dry hop.
In Primary: all-grain ESB
In Secondary: 2011 Szechuan and Black Peppercorn Mead
Bottled: 2010 Clover Mead aging for 1,3,5,10 and 20 years
25+ batches 10+ years ago and back into it with 10G all-grain beers and annual meads. Equipment: DIY cooler mash tun with slotted copper, DIY keggle and keg-HLT over propane, DIY counterflow chiller, Mini-Brew conical fermenter, 4 Party Pigs. 1999 AHA Nationals- 1st Place Herb and Spice Mead "Szechuan Peppercorn Mead".