Yes, to keep all the holes in the same plane you first ensure the fixed vice jaw is parallel to the mill access with a dial indicator. Set say the bottom face of the work against the fixed vice jaw, then turn it 180deg to do the other holes keeping the same bottom face against the fixed jaw
Although the Bridgeport has the ability to take the metal out with one cutter the full width if you are buying two cutters then 6mm and 12mm will come in useful but as its mostly RC work you are doing then 5mm and 10mm may be the better option.
Assuming there is about 30mm width to come out the middle then 3 passes of your 10mm cutter at 2mm deep each pass should be fine, use a bit of utting fluid – parrafin or WD40 will be OK on alloy
The thing you describe is a rotary table, could be used to do the two semi-circles. Set the RT axis true to the mill spindle, clamp the work to the table with one of the mounting holes aligned on teh axis. Then you can use your 10mm cutter to forn the semi-circle.
Take a look at
this thread by John (Bogstandard) the first couple of pages detail how to set up your machine, make your block of metal true and then how to machine it.
For tapping I would suggest drilling the 3.3mm hole and then without moving the mill take take your tap with the tap wrench gripping it half way down and place that in the chuck but don’t fully tighten, just enough to hold the tap true, you can then cut the thread without risk of the tap going off at an angle (DO NOT USE POWER) This also helps prevent a snapped tap as the tap wrench will rotate before the tap jams and breaks
J