For number 1, the irb interface will be down until one of the physical member interfaces shows link up. Perhaps this is the issue?
I'm also wondering in general, is the SRX100 configuration up and running correctly now?
If so, can you start by exporting this configuration and just changing any interface naming to the new designations on the SRX300 and importing the working configuration as a starting point?
It might be easier to make modifications from the known good instead of starting from scratch.