1) tires
Yes, you can make 20.000 with one set of tires. Depends on the tire of course and on how tires wear on your bike generally.
On my R 100 GS a set of Metzler Tourance the rear tire lasted about 25000-30000 km, the front tire about 20000-22000 km. For Details see
ontheroad | Resis Zipperlein (German only)
I heard good things about the life expentance of the Mitas E 07. Ask beddhist on this forum. As far as I remember, he rode about 30000 km on his Mitas as well on his DR 650. Cannot tell myself, just bought my first set.
However, once you are in India, you will need tires as well and it is not easy to get them there, since India charges 500% custom fees on imported goods (afaik).
It is still possible to get tires (might be difficult - I had a hard time to find an 21" tire in Delhi, was not too bad though), but do not expect international brands!
For that reason it might be sensible to carry spare tires anyway. If you carry them yourselve, your Carnet is valid for them as well (there is an extra section for spare tires in the carnet but I never bothered to note my tire and nobody complained)
2) Visa for PAK
Consider that Pakistan and India are foes, so I expect it would be as difficult getting a Pak visa in Delhi as it is a hassle getting an Indian visa in Delhi. Again, I did not do it myself. I entered Pakistan within the last days of validity of my multiple entry visa and extended my stay in Lahore.
You might consider this: The visa for Pakistan is easily extendable and the extension is free. However, they will extend only your current stay. So even if you entered the country on a multiple entry visa, it will be expired if you leave the country on an extended visa. HTH
3) Temperature in October
Yes, it will be cold, even further south. The mail road from Bam to Esfahan is as high as 2500m.
I entered Iran by mid-november from Pakistan and I had snow on my way from Yazd to Esfahan.
As far as I know, snow is a major issue in the border region between Iran and Turkey a bit later in the year, too. But I also heard that you can put your bike on the train as well if the conditions are too bad.
Again, ask beddhist. He entered Iran from Turkey by mid-octobre afaik.
HTH
Lars