Q1: Yes, the giant purple crystal is the biggest you can get by killing 10 enemies+ in order, not an easy feat.
Q2: It probably does something to the lemons if they're alive, but since he killed tham all before-hand, I haven't a clue. Any pros got an idea?
Q3: They're Raspberries, only seen in Hell. I'm not going to give any info on what they do as I've never encountered them myself, but others will know.
Q4: If you collect

, then they fall. Same with

and the yellow one, dropping the respective coloured crystals.
Q5: I honestly don't know, but i'm guessing it's something to do with transforming the enemy instead of killing it, therefore not counting as killing it in order, but that's just a guess.
Q6: Not at first, but if you get to level 49 in 2-player mode, you get an item called the shared heart, allowing you to share lives permenantly.
Q7: I believe it may do, since I have had the same trouble when having 8 lives. Also, I remember once getting CRISTAL twice in quick succession (8-10 levels difference) with only 2-3 lives. It may also increase the chances of a Gold Igor appearing, but that's just my theory.