I guess we should ask if the bolts go all the way through both top and bottom of the tubes (that is what we are assuming and seems likely), or if they only go through the top sides of the tubes with the nuts fished inside. (When I replaced the std. coupler on my boat trailer I used individual bolts on each side and another down through the top with nuts and washers on the inside of the tube.)
I believe that that style of coupler is intended to be welded on and does not come with any holes, so I don't think that it is as simple as filling all of the holes. I could be wrong and hope for Cathrine's sake that it is that simple.
The formed shape of it is inherently strong, with the formed lip around the bottom edge of the two sides (as well as the ball socket) adding significantly to that, so is of similar strength to the two piece plate clamp being eliminated (... at least in my "eyeballing thumb at arm's length" opinion). The coupler itself is miles ahead of the one being replaced, as far as strength goes.
Cathrine, I see no reason why it would not be fully adequate for the planned end use provided that the bolted joint configuration is adequate (grade 5 fasteners and crush sleeves are recommended). Is it a 2000 lb rating or higher?
An additional bolt through each side is probably not a bad idea, so long as the three holes are not all too close together in the tube... wouldn't do much good to turn it into Swiss cheese. The bolts that have been added look to be fairly close together and I wonder if there is enough remaining tube under the front section, the pointy part of the triangle to get another bolt through the tube without being too close to the front end of the tongue tubes.

Maybe there is enough room to the rear.