Wow, that's a lot of questions Breezes!
I'll try to address as many of these as I feel comfortable answering. I haven't studied as extensively as some of the other members of this board, but I can offer you at least some guidance.
As for your question about spreads, you're partially right. If you laid all the cards in a straight line they'd have the same meanings as the spread numbers, but spreads have a layout for a few reasons. Firstly, like you said, some cards can be read in relation to their position near other cards. If you feel this is too advanced for you at this point, my best advice is to stick with smaller spreads between 3 and 6 cards. The Celtic Cross is 10 cards, but its very easy to learn and a lot of people start with that one too.
Another reason spreads have layouts is because many times it helps to remember the meanings. The Celtic Cross is a great example of this. In the Celtic Cross, the cards going from left to right in the first section are past, present, and future [Cards 5, 1, 6]. From top to bottom they are conscious (influence you see), immediate influence (you may or may not see), and subconcious (something you aren't seeing) [cards 3, 2, 4]. The stem of the cross (which is placed to the right of the first half, only to save space), reads from bottom to top and goes: Queren'ts feelings, Outside Influences, Hopes and Fears, Outcome [7, 8, 9, 10].
here's what that spread looks like. Card 2 gets laid sideways over card 1.
---3------10
5-1/2-6---9
---4------8
----------7
Do you see now how the logic of the layout works? It's easy to remember the card positions because they follow closely with the meanings of positions next to them. (e.g. past, present, future). So the spread can be read in that manner.
Ease of remembering meanings and why each position has those particular meanings is probably the biggest reason we have spreads. If we laid every spread out in an easy shape, like a square, or a line, it might take more time to remember how each one differs in meaning from the next. However, with most smaller spreads, the shapes are very simple, and it works just fine.
And of course, different spreads adress different problems, and therefore are designed to give different answers that pertain to your specific situation. Some spreads will tell you about relationships, others about a job, and others about past lives even. Each spread has specific meanings so that it can adress a problem more directly and accurately, rather than just using a general spread to get general feelings about the situation.
As for your question on reversals, many tarot readers believe that if a card is laid upside down in a spread, that it has an alternate meaning. However, there are also many readers who don't practice this, and especially beginning readers. I have been into tarot for a few years now and I still don't feel comfortable enough with the card's meanings to learn how to read them reversed yet, so my best advice is: For the time being, just get comfortable with each card's regular meaning. Don't worry about cards getting turned upside down while you shuffle. Just lay them down one at a time when you do a reading, and if you lay any down upside down, turn them around, so that all the cards in the spread are upright.
That's the way I do it, and it works out just fine for me.
About significators. The Significator is usually a Court Card, but can be a Major Arcana card too. This is simply a card you pick from the deck before hand that you think best represents the querent. I personally have never fully understood this practice either. I think it's purpose is simply to set the mood for the spread, but if you take out a card as a significator, then that card can't appear anywhere else in your spread, and I feel like that can make you miss a valuable oppurtunity to get answers from that card.
That's all I can help you with. I'm sure some others on here can chip in on your other questions and add to some of my responses as well. My best advice is to just take it slow. Stick with simple spreads and learn the cards. Read the meanings often. Thirteen's Tarot Meanings on Aeclectic really helped me to learn them fast.
Here's a link to that.
http://www.aeclectic.net/tarot/learn/meanings/
Good luck with your readings Breezes!