Hi,
i am also new... but this is easy to answer :-)..
a) yes, you can "chain" different sequencer-parts together. Look into your manual under cuelist... basically you just store the same sound just with different sequences in different programs... With Cuelist you can switch seamlessly between them. Some user reported to not switch with the programdial, but with the 16 Trackbuttons of the sequencer. This avoids unwanted rythmical issues while switching... one more thing: you cant run a whole song with that. You have to switch the sequences handish... easy for a drummer :-)
b) generally yes, you can play on top... the thing is: if you have a monophon sound it will stop the sequenced note... there are possibilities to play on top in paraphonic mode... stil you have just one VCA (which means, you will maybe cut off the sequenced sound when playing on top of the melody)... just try out a little, but i would recommend to either play the pro 2 totally live or run a sequence...
last idea to your question. there are some relativ cheap sequencers on the market, that offer more pattern features... i personally use a MPC live to sequence midi notes into pro2, that will sequence the pro2 sequencer..:-)) sounds weird, but gives nice results...
anyway... have fun... its the best synth on the market (at least it is for me :-))