Q:How to control early reflections?
This is a simple but very effective and good sounding trick. Just use the volume envelope to adjust the head of your impulse response.
Q:Are SIR2 presets compatible to SIR1 presets?
No, because SIR2 is a completely new development.
Q:What are the differences between SIR1 and SIR2?
Please look here.
Q:So, why SIR2 doesn't have the attack, envelope and length controls like SIR1?
Simply use the amplitude envelope to simulate these parameters. The straight lines in SIR2 will have the same effect as the curved envelope in SIR1, because the waveform display of SIR2 utilizes a logarithmic amplitude scale. The amplitude envelope in SIR2 is much more flexible than the few dynamic controls in SIR1.