Kindly clear few queries regarding Automata theory !

Question

I'm studying through Linz and Have few confusions :
 

1. Let $\sum = {a, b} then \sum ^{\bigstar} = \epsilon ,a,b,aa,ab,ba,bb,aaa,aab,..... Here can we take ba ? I  mean placing doesn't matter ?$
2. On page 74 second para its given (a+(b.c))^*  is the language {$\epsilon ,a,bc,aa,abc,bca,bcbc,aabc,..... So can we take bca here? I mean placing doesn't matter ? please clear$
3. Its said a regular grammar is linear but how because there is no restriction on position in linear but in regular its either left or right means a restriction ??
4. its given at some place L = {w $\in$ {a, b}$^\ast$ : n_a (w) = n_b (w) and n_a (v) = n_b (v) , where v is any prefix of w } . I'm not getting this whole notation ,please explain thank you !

Comments

Interpret + as or and . as concatenation.

So you can read (a+(b.c))* as (a or bc) repeating 0 to infinite times as

(a or bc)(a or bc).... 

3) Refer this https://en.wikipedia.org/wiki/Linear_grammar

4) v is prefix of w. Hence the word is of the form vw 

n_a (w) = n_b (w) number of a's in w = number of b's in w

n_a (v) = n_b (v) number of a's in v = number of b's in v

ϵ (no a's no b's)

ab  (1 a and b in v, 0 a and b in v)

ba,abab,abba,baab,baba,... 

You can't have aa,bb,aba, etc

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thank you MiNiPanda :)

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 thank you gauravkc :)