It’s no more than couple of hundred yards. Walk to the front of tour train, then carry on to the escalators , ride up, walk across to 11 and ride down. Tickets are only sold for reasonable connections - 10 minutes is enough I can’t put links in here but google “reading station map”

No 1st class ones

National rail app is very good.

Hi it depends on the time you're going. You can check on nationalrail , put in Reading to Cardiff and then select the time, the search comes up with all the times and platform (some trains say platform 12 rest are platform 9). But always worth checking when you get to the station as they often have platform changes due to rail works- or if a train breaks down/ train delays on other services

To Be Honest I really Dont Know It's not something I would normally look for, usually if I'm staying at a hotel I can leave my luggage there before traveling.

Yes - there are waiting rooms

Yes there is always black cabs on the rank

It's less than half a mile, so 10 minutes should be plenty of time