Thanks.Yes, the central bearer is positioned horizontally, i.e., the wide faces are parallel to the ground. You could certainly strengthen it by adding a piece on the underside (glue and screw), but I really don't thinks it's needed. I say that because in the case of one the beds I've made the mattress supporting slats are 20 mm thick poplar (tulipwood) on top of the ~60 X 32 mm poplar longitudinal bearer carrying an 80" X 60" (2030 X 1520 mm) mattress, and after nearly twenty years there's no appreciable sagging of either the slats or the longitudinal bearer. This bed is in our bedroom, it being being one of a pair I made, one for a customer and the extra one for us. There's also no additional middle leg, and time has proved it's not necessary, but if you want to include one on your bed, that's your choice. Adding one does rather clutter up the under bed space, but it's your choice.
I wouldn't choose your side rail connection method. If I was going to put a bolt of some sort in I'd want the bolt to pass all through the leg. With yours there's a small chance the brass threaded insert could pull out from the leg leading to looseness at the rail leg intersection. In addition you need a spanner to assemble the bed, whereas with the Noval K type fittings the two parts lock together as the side rail hook part drops into the leg mounted socket part. In general I'm not a fan of bolt together beds, but that's just my personal bias. If it works for you and you're happy with it you'll be in line with a lot of other people that have used the same assembly method. Slainte.
I have decided not to use through bolts because I can buy here only cheap-looking bolts (zinc-galvanized no brass other than the hexagonal head). I think that your Noval K fitting is easier to assemble and less work is also required to install it on the rail/leg than my method. What is the correct procedure of installation to ensure that there is no gap between the leg and rail for this type of connector? Shall I screw the first part to the leg, then assemble both parts of the fitting together and then make holes for screws on the side rail when side boards are attached tightly to the leg?
Is there a requirement that either side rail or footboard or headboard boards must be flush with the inner corner of the leg so that there will not be a big gap after I add the mattress? If not, I could use Noval K fitting, because it is less work for me, provided I can make the resulting butt joint tight.