Can those 2 holes in a CLU048 1212 be used to mount it?

I see in most builds people use Ideal holders, however if enough care is taken (dont screw in too tight), could we use those 2 holes to screw into the heatsink?

If not, what are the holes for?

Also, does BJB do a holder for the 6th Gen 1212's? The reflector-type-lip design looks more efficient than ideals design.

Yup, to all your questions. The holes are there so you can screw it in. People like using holders because it eliminates the need to solder anything. And the BJB holders are the same as the Gen 5. The dimensions didn't change. Just watch out for the 150v rating on BJB's.