Especially in 4x4 vehicles, a two piece Tail shaft is best.
Most custom conversions to a one piece suffer when both rear wheels lift off the ground because the tail shaft usually hits the cross member because it is so long. This can happen easily over speed bumps & if a vehicle is bogged to the axle.
One piece tail shafts also get a flex wobble/...
If they are matched brand across the axle you'll be fine for the sort of driving you describe - assuming the new rears are of comparable quality to the fronts. If you put ditchfinders on the rear you might find it a bit unpleasant in the wet!
Regarding sizes - the OP mused in the comments about putting 225/50/R16 on the rear with 205/50/R16 on the front.
The fast/smooth part you're asking about is because there is a nearly continuous flow of power. In a normal standard shift, where the driver presses a clutch pedal to release the clutch, it also releases (or disconnects) the power between the engine and transmission. In a DCT setup, the computer is ready from one shift to the next. It has the "next gear" ...
No, it's not safe. Not in my opinion anyway. That could seize or break at any time causing an accident involving just your vehicle or other vehicles.
It looks to be far beyond reparable condition. Just get a new driveshaft installed.
If it is winter and you have summer on the back and winter on the front then you will definitely notice. Summer all round and not four wheel drive will be ok.
If it was 4 wheel drive then keeping the tires matched is important depending on the 4wd drive system - and Volvo is one known for that.