1) Yes you can use the pressed HDA or better yet get a mod chip and use the HDL that is free and put in on your memory card and with a chip like the Matrix Infinity you can boot HDL from your memory card or HDD with no disc in your PS2 at all.
2) Yes the compatibility with HDL is very high, have a look here for a list http://list.ps2hd.com currently at 72.8%
3) yes there is a no solder, swapless chip called the DMS4 EZI but its known to be a console eater with its clip kit, the PS2 may get damaged and un-repairable (yes I have seen this first hand with many of my customers) Consider the Matrix Infinity and have it soldered in.
