I don't see why Mindleech Mass wouldn't include creatures. Magic is all about reading text carefully, and there are cards such as Negate that specifically say "Noncreature" in them.
One way to look at deathtouch is that vs a creature with deathtouch they have a toughness of 1. That is simplifying things to the extreme though. In regards to Guard Gomazoa, I do believe it still dies because even if combat damage isn't being dealt the creature with deathtouch is still "touching" him, and any "touch" kills.
Each Drowner Initiate is a separate ability, because they are on different physical cards, so you would have to pay 2 colorless mana total to activate both. In addition to the cost of the blue spell of course.
I'm not 100% sure on the shroud, however I believe that counts as an ability and abilities are only active when they are on the battlefield. So I think once in graveyard all bets are off with shroud. You may want to call a local comic shop to be sure on that though, but I'm 99% sure that because they are "dead" their abilities don't work. Although I'll admit there are some abilities you can play from the graveyard...
A better question than how much land should I have is what type of deck do I have. If you're running mono (one color) then you can probably get away with 18 total. You can split that up however you like, all whatever color mana you're running (ex 18 islands for blue) or adding in fetch cards like Evolving Wilds, Scalding Tarn, etc. Or maybe additional effect cards that like cause your opponent to lose one health (Piranha Marsh, which is black) or a creature gains flying until end of turn (Soaring Seacliff; Blue)
For a multicolored deck you want at least 20, and most I've seen have 22 including some fetch cards. Fetch cards get a little more important in multicolored decks because you're now dealing with two different mana colors to juggle and say "ok, I need a blue mana so I can cast this, but I have no islands in hand... But I do have a fetch card!" So then you could get an island and cast the blue spell. It also helps thin your deck some so you have a better chance of drawing what you need.
2+2 is of course 22, which is how much land you'll probably run even in a mono deck, just to be sure you can cast things. plus there are generally some nice cards in each color that play off how many land you have that give that color mana. For example Landbind Ritual, which gives you 2 life for every plains you control. If you have 10 plains out (and it's not uncommon in mid-late game to have that many lands out; assuming you aren't going up against a super quick deck where you lose in 3 turns) you just gave yourself a huge life boost.