If you've reached "quota" on the 1st round, then yes, you automatically get selected.
They are not "scored" in any way: the different preferences are not "worth" more per se.
If no candidates reach quota, the "least" voted for as a 1st preference gets his/her votes reallocated according to the 2nd preferences for those that had voted for them.
Once a candidate has reached "quota", only those votes above the quota get re-allocated (I think - not sure how they work that out)
No you don't need to rank all the candidates. If there are only 3 or 4 you like/can thole, only rank those 3 or 4. In fact, you shouldn't rank people you don't want, as if the candidates you do want get eliminated in the first few rounds, then if you've ranked more even though you don't like them, your vote will count for them (say your 1st 4 preferences don't reach quota, bit you'd put down 5 even though you don't like the 5th, you'd effectively have voted for that person
).
I think it was in a by-election that Nicola Sturgeon's dad won the first round but didn't reach quota and it was only in the sixth round that the Labour candidate won (ie 4 other candidates had been "eliminated" and their next preferences allocated - and it was the Conservative votes then being allocated to Labour that won them the seat)
Because you are voting for a person, not a party, if you want all the candidates for that party to be elected, you need to rank them in your top preferences. There is a disadvantage for people with surnames towards the end of the alphabet, as they are ordered alphabetically on the ballot paper I could never stand as I'd be at such a disadvantage