Can a 2" bore pipe into the silencer be welded up to/clamped to,lengthened and bended to meet the back of the mid section on my brothers car (he will only be using 1 backbox including the 6x4 oval-2 1/2" short pipe-back box-2" bore mid secion connecting pipe)
Yes it can, it would help to know what car your brother has as every car is different.
And if i wanted to quieten down my system,would welding a 3" bore 10 inch long pipe in the middle of the mid secion (so it gets wider for 10 inches then narrower again) will that silence it a bit?
Nope mate, you need to weld a "Bullet Silencer" in there.
Costs vary on every vehicle as every vehicle is different fitment. As a guide, £30 per hour labour plus brackets, clamps, reducers, mountings and consumables...