1. All are safe - maybe with the exception of LA - probably only truly dangerous place on your list. Italy is also dodgy some places, beware of pickpockets. Less so in Prague and Budapest, but keep common sense with you in tourist areas
2. There are enough decently priced small hotels and guesthouses, there is no need for a hostel in Bangkok. Bangkok is very safe - but keep an eye out on the flooding situation
3. Yes, you have access to all lounges, but not all are created equal - best one will be in Istanbul
4. Depending on your connection timing - you may qualify for a free tour of Istanbul from Turkish Airlines - google it.