Can every linear grammar be converted to Greibach form?
The more general answer is:

Blum and Koch showed a polynomial time transformation such that any context-free grammar can be converted to Greibach form.

Since a linear grammar is a special case of Context-free grammar, the answer is yes.



A simpler transformation:


Any rule $X \rightarrow a_1 a_2 \cdot a_k Y$ you transform them in $k$ rules: 


$X\rightarrow a_1 X_1Y$.
$\cdots$
$X_{i-1}\rightarrow a_{i}X_i$
$X_{k-1}\rightarrow a_{k}Y$

Any rule $X \rightarrow a Y b$ should be transformed in two rules


$X \rightarrow a Y Y_1$.
$Y_1 \rightarrow b$



where the capital letters belong to $V$ and the small letters to the alphabet (terminals).