[susan53]

1. Juxtapose = place next to each other. Here are three genuine pieces of spoken English (from Crystal and Davy, (1975) Advanced Conversational English, Longman
a. ..it was lovely our one with the nightclub (p.52)
b. Sussex has super heath country lovely lovely sort of rolling heaths (p.80)
c. they'd be off and away little grey smooth sleeky things (p58)
The idea are simply placed next to each other.

Co-ordinate = link ideas together with a conjunction such as and, but, or :
..it took us 40 minutes to catch them and all we had to do was to get them out of the cage and into the bucket with the lid on but they were - they just went like this and they - you'd go you know and they'd shoot out through the bars of the cage and they'd be off and away little grey smooth sleeky things and we used in the end we devised a very good trap.... (p.58)

2. No. As I said above, the relationship between these two ideas is concession - a change in the direction of the discourse. So you need a concession connective like but. And is an addition connective. It indicates that the two idea are the same and therefore the discourse is continuing in the same direction. Compare :

a.I don't like vegetables but I eat them because I know they're good for me. If someone says I don't like vegetables, you expect them to continue so I never eat them . Here however the discourse changes direction and "surprises" you with the fact that s/he eats them all the time.

b. I love vegetables and I eat them every day. Here the discourse continues in the direction you'd expect it to. The two ideas are "the same".

So with the Mary example, consider these ideas :
She isn't very kind = negative trait
She's lazy = negative trait
She's quite smart = positive trait
So :

A : I don't think Mary is the right person for the job. B : Well, it's true that she isn't very kind - but she's quite smart.
B accepts that a negative trait exists but then changes direction to propose a positive trait. So the two ideas are different : negative /positive

A : I don't think Mary is the right person for the job. B : Yes, I agree. She isn't very kind and she's lazy too.
B states two negative traits. The two ideas are the same : positive / positive