I will be arriving into Delhi T3 on RA205 at 9.40 then leaving at 21:00 on AI602 from T3 as well. I have star alliance gold. Will I be able to use the lounge for the 10 or so hours between flights?

No, you can't enter the domestic terminal until about 3 hours before flight time.

I suggest going to the AI ticket office to request flying standby. Not sure if AI allows this without fee though.

You can pay for the arrivals lounge (3 hours) but with such a long layover I strongly suggest an airport hotel or rent a taxi for the day and do some sightseeing.

My recent experience:

It is a pain to transit from International to Domestic even if both the flights are from T3. Going the other way (Domestic to International) is relatively better if you have a boarding pass in hand as you can walk directly to the immigration desk.

Once you come out of international, you have to recheck your bags for the domestic flight. I have always seen a long line there in what seems to "Welcome to Inefficiency" desk. During the most recent transit, I walked past it and to the end and took an elevator to the departures after passing 2-3 security checkpoints. No one stopped me because of the 3 hr rule (once inside the airport building, I feel this is not imposed as much).

After checking in, I went to the lounge (used Premium Lounge and not that of AI) because I have a couple of cards that give me access to it. If, by chance, you cannot get past security and check-in, there is a Premium Lounge as soon as you get past the Customs (to the right) that seems fine (advertised "showers available") and is much better than what's available past the check-in/security.

transiting from international to domestic is a different scenario....the 3 hour rule is a bit relaxed in this regard....if you come across a helpful security personal then they will allow you to enter the domestic terminal right away....

I've ended up getting a room at a hotel 2 miles from the airport for �2 just in case.

Don't particularly want to be sat around a transit area for 12 hours.