For 1, if you keep rotating the tyres every mile or so, you get equal wear on each.
A kilomile eats 2/42 + 2/48 = 5/56 tyres, so 56 kilomiles would eat 5 tyres exactly
For 2, we have three changes, so one of the tyres won't get a turn in the trunk. Therefore, the answer cannot be more than
So if we find a way to run each of the other tyres for that long, we are good.
Turns out it's quite possible:
Keeping one tyre in the front position, and rotating the other four every 12 kilomiles, each of the four tyres accumulates 24 kilomiles in the rear, 12 kilomiles in the front, and 12 kilomiles in the trunk. That makes for a total of 24/42 + 12/48 = 23/28 wear on each of the four tyres.
That solution has the added perk that you can continue on the remaining four tyres after the other one goes. If you don't care about that, here's another way:
After the first 6 kilomiles, swap the left rear tyre with the one in the trunk. After another 6, repeat with the right rear tyre. At that point, you have driven 12 kilomiles and have 36 kilomiles left in all 4 installed tyres.
The benefit of this other method is having balanced wear in the front all the time, and, except for the bit from 6 kilomiles to 12 kilomiles, in the rear as well. Also, after the first 12 kilomiles, the better pair will be the rear one, which is the recommended way of installing an unevenly worn set of old tyres. Since we are going to wear the tyres completely out anyway (not really recommended), this at least improves the survivability of the journey as much as possible.