First, Neutral axis. If you imagine a joist 10m long then place a wight in the middle of that joist, the joist will deflect downwards, slightly, in an inverted curve. If you then measured the length of the top surface it would be a little under 10m as the top surface would be slightly compressed. Likewise, the bottom surface would be a little over 10m under a slight tension. In theory, therefore, there is somewhere on the joist depth that is still 10m end to end and is, therefore, neither compressed or stretched and is, therefore, not absorbing any of the load placed on the joist. That's the neutral axis (and before I get shot down, yes this is a qualitative explanation to explain the situation ignoring the implications of Newtons 3rd Law and the resulting couples of inertia a load on a beam creates).
The result of that is that if you drill on that axis, you are reducing the load bearing capacity by a minimum amount, which, for practical purposes, means if the centre of your hole is exactly half way between the top and bottom surfaces of the joist you've hit the neutral axis. If you can use the existing holes, use them - you are maintaing the structural integrity to the best level you can.
I still have a very old metal CU which when the cover is off, the live contacts on the main swith could be touched.
Then don't take it off!!!
Yes, you can route cables through the same hole. If it's more than 2 (max 4 in the tables) the load carrying capacity is reduced by a couple of amps as long as the cables aren't surrounded by insulation.. This, on average, isn't significant.
I wlways, where possible, clip cables to joists. I get this from my father, an ex-naval artificer electrician who started domestic work upon his demob in 1947. His view was that neatness and precision go hand in hand. I reckon clipping, where possible, minimises the possibility of damage to the cable at some future point, though some may disagree. The only actual "rules" are based on supporting the cable by this method that prescribes clips to be a max of 250mm apart for other than the larger cables.