2. 506 is the only all CLRV route. 501 is mixed, and the rest are all Flexity now. 505/511/502/503 are currently running with buses, and you can catch the oldest buses in the fleet on these routes.
4. The busiest bus route in the downtown core is 6 Bay, keep it mind that since it mostly serves M-F commuter traffic it's very frequent during the day but not on evenings and weekends. South of Bloor the 29 Dufferin (which has Novabus artics) and 63 Ossington are busiest and most frequent bus routes, but they are a bit outside downtown. Everything else downtown is either peak only (the 14X express routes) or lower frequency services like the 121, 75, and 72A routes).
You'll also be able to catch GO transit buses (MCI D4500 and Enviro500 double decker) coming in and out of the Union Station bus terminal, generally north on Bay and south on Yonge from the Gardiner Expressway.
5. My favourite stations are Davisville (outdoors and next to the Davisville Yard) and St. Clair West (1970s brutalist architecture and a pretty big underground bus and streetcar terminal). You also might want to check out Finch, it serves the Finch and Steeles bus routes, some of the busiest and most frequent in the system (Finch East has <2 minute headways in rush hour) and suburban YRT and GO buses. Also, technically not a station, but the Queen/King/Roncesvalles intersection and Roncesvalles carhouse (try to go in the early afternoon when the PM rush runs leave the carhouse).
If you have the time, you might also want to visit Humber loop (where the Long Branch section of the 501 meets the main section) and ride all the way to the end of the line at Long Branch loop. This is part of an old interurban route, it takes you all the way to the city boundary and has a very different feeling (almost like a small town main street) than the rest of the streetcar system.