What is the mapping between the number of thread and csv_data

i doing jmeter testing. setup is:

thread group: 1 threads in group: 10 loop count: 5

i have csv data config file contains:

usr1, passwd1 usr2, passwd2 usr3, passwd3 usr4, passwd4 usr5, passwd5

'recycle on eof' set true, 'stop thread on eof' set false.

a couple of doubts:

i think setting 'recycle on eof' set true, thread 1-5 use username/passwd 1-5, thread 6-10 should reuse username/passwd 1-5. wrong, thread 6-10 eof username/passwd , fail test obviously, article suggest. but why?
then set 'threads in group' 5, problem in 1) gone, thread 1-5 use username/passwd 1-5 loop 1. because loop count>1, not know when thread 1-5 finish 1st round, username/passwd pick loop 2? should thread 1-5 still use username/passwd 1-5 in order always? observation it's first come, first serve thing, whichever thread finish loop 1 first username/passwd 1. expected behavior?
for reason, test not allow 2 threads run using same username/passwd @ same time. if describe in 2) true, can have below timing situation:

a) thread 1 (use username/passwd 1)takes longest time

b) thread 5 (use username/passwd 5)takes shortest time

c) when thread 5 finish loop 1, grab username/passwd 1 , run, @ moment, thread 1 loop 1 still in progress, bad things happen...

so configuration can set have threads wait each other finish current loop, start run next loop together?

if have comments, please feel free share. thanks,

Ant COmde