I also took the metro from Lisbon airport to Rossio Square, close to where I was staying. I did have to change metro lines once, but it was not a big inconvenience for me - I am used to public transportation.
I visited Sintra on a Friday in May. According to Rick's book (I think?), weekends are busy in Sintra with locals and that Mondays are busy because museums in Lisbon are closed. I took the train from Rossio to Sintra, and yes, it was pretty crowded on the 434 bus from the train station going up the hill to the palaces, but I was able to get right on the 434 just after I got off the train, so no waiting. You could always hire a "tuk tuk" little 3-wheeled taxi to take you up the hill instead - can't cost that much.
I had not planned well and did not buy my Pena Palace park tickets online ahead of time, so I had to wait a long time in a ticket line at the gates. Besides buying them online, I later learned that I could have bought them at the Moorish Castle too (much less crowded than Pena Palace) - so I would have done better to visit the Moorish Castle first visit and buy tickets to Pena Palace, then go on to the Pena Palace park and walk right in.