@Muthu.svg bro, those are some good questions. Let me try answering them one by one.
You can read my Airtel Black (fiber + iptv) connection review thread to get insights on the same.
Bought Airtel Xstream Fiber Black (Fiber + IPTV) connection with AOT-5221ZY (wifi 6) ONT
I use Airtel Black combo (fiber + iptv) and I find it worthy because I get more channels compared to Tata Play at reduced cost with 200 Mbps internet.
The speed reduction due to IPTV box is negligible because it uses 2-3 Mbps bandwidth maximum.
Airtel has curated packs for bundled fiber + IPTV plans. You will get the default pack channels initially but once connect is installed you can add more channels directly from STB or via miss call or Airtel Thanks app.
No, they won't do ODU DTH setup because Airtel has stopped providing DTH for bundled wifi connection after IPTV service launch.
Only internet based IPTV box is given by Airtel for bundled connections these days.
It is rain proof, unlike traditional dish antenna based DTH. So you would get less interruptions compared to DTH even during rainy days.
Since dish antenna based DTH is not provided now, this question is not relevant.
For IPTV service, if fiber internet goes down, the TV also goes down.
Only one channel difference, Star 4K channel is not available. Rest all things are the same.
Furthermore, the IPTV box is more advanced and having modern hardware & software compared to Xstream DTH box.
Please let us know if having any more questions. Our forum members would be able to answer.