For the 9h11p9 thing, the 'h' means hammer on, and the 'p' means pull off. For this sequence, you're suppose to hold th 9th fret, probably with your index finger, pluck that string, and quickly hammer your ring finger on the 11th fret, and just as quickly pull the ring finger down a bit and then lift off so you're kind of pulling the string and then let go. It'll sound like a quick 'da-la-da' with only a single pick at the first 'da'.
A riff just short rhythmic phase in a song, usually a melodic solo, but sometimes it can be a background solo. If you're talking about the intro riff, it's just the melodic strumming that you hear at the beginning of the song. It's just the root chord of that section as a whole is Asus2.
For the strumming of the intro riff, whether if you're doing just the root chord or the riff chords, it's pretty simple and straightfoward. This is the basic pattern:
D means down strum, and U is upstrum. The When you play, emphasize on the 3rd 'D'. So you play pattern for each of the 4 chords of the riff, so totally you play this 4 times.
Then you get into the main strumming pattern:
Another second common strumming pattern is:
For the 2nd line of the intro (after the riff), you need to play the whole line twice, and you should strum like this:
Asus2 | Bsus2 | C#m | Bsus2
D-D-D--UDUD-D-DU | D-D-D--UDUD-D-DU | D-D-D--UDUD-D-DU | D--UD-D--UD-DUDU
(each repeated pattern and chords are separated by '|'. It's hard to do the spacing with this font.)
For this line in the verse, "E Asus2 C#m Bsus2 (X2)", you need to play this twice and play the main strumming pattern twice per chord. So it's kinda like this:
E | E | Asus2 | Asus2 ...
D-D-D--UDUD-D-DU | D-D-D--UDUD-D-DU | D-D-D--UDUD-D-DU | D-D-D--UDUD-D-DU | ...
For the second half of the verse, "F# Asus2 Bsus2 Bsus2", just play the 2nd strumming pattern twice for each of the first 3 chords, and then one drum to ring out the last chord, like this:
F# | F# | Asus2 | Asus2 | Bsus2 | Bsus2
D--UD-D--UD-DUDU | D--UD-D--UD-DUDU | D--UD-D--UD-DUDU | D--UD-D--UD-DUDU | D--UD-D--UD-DUDU | D (let it ring out)
Then the chorus is uses the main strumming pattern once again, but just one pattern per chord:
Asus2 | E | Bsus2 | C#m
D-D-D--UDUD-D-DU | D-D-D--UDUD-D-DU | D-D-D--UDUD-D-DU | D-D-D--UDUD-D-DU
And you need to repeat this 4 times.
That's pretty much it ... for the main stuff. As for the solo and interlude, I have to relisten again. I'm pretty sure interlude (the power chords section) is just down strums.