To tell the truth I've never had the need to make a multisession disc but i believe I can answer your questions.
1)Cant say for other programs but Nero can do multisession for write-able DVD's, just start Nero Burning ROM and you will see the multisession option right in front of you.
2) The Philips drive probably already sets the booktype to DVD-ROM you can check it by putting in a burned DVD+R disc and start Nero CD/DVD Speed (make sure you have the latest version) and click on the disc info tab and you will see a booktype field and see if its set to DVD+R or DVD-ROM.
3) Never tried but cant see why not.
4) You can crossflash your drive to the Benq 1620's firmware which will allow you to do quite a variety of new things like burning DVD-R, over speeding media and disc quality scanning but by doing do you invalidate you warranty.
Go to this website download WinDWFlash and the B7T9.cvt (plus also download the B7V9 firmware) now using WinDWFlash load in the B7T9.cvt and flash it to the Philips drive, after the computer reboots you can then flash the B7V9 firmware (which is the latest) with BenQ's own flash tool.