Need advice -- some phrases

Hi, I'm trying to post this thread again since my previous attemps did not succeed.
I really hope that this thread will be posted! Would really appreciate it!
I have a list of very short questions. Could anyone please check whether the following sentences/phrases sound ok:

1) we set these parameters equal to 1/2 of the error value.

2) The model allows describing the process...

3) Can I write 'and' after a semicolon in the situation like this, or do I just need to put a semicolon without 'and':
x+y+xy-ab=x,
where x is the number of the mutant cells; y is the component of dependence; and ab is the parameter of something else.

4) Therefore, to describe these parameters, we need to bla-bla-bla (isn't better to write 'in order to describe'). Is this construction totally wrong?

5) We need to put x=0 (is it better to write 'to set')

6) Do I need to set a comma before 'or' in enumeration like this one:
the parameters were modelled for the three types of cells in the XX, YY, or ZZ gene.

7) Where is it better to place an adverb 'simultaneously':
The mathematical procedure consisted in simultaneously (?) running simultaneously (?) the models for XXX, YYY and ZZZ.

8) Can I say "That is the main reason why..." Isn't better to say 'that is why' or 'this is the main reason why bla-bla-bla'

9) Do I need a comma here:
The results are three times more important than for the XXX cell, just like in the experiment.

10) Besides our previous study, only one paper is concerned with simulating these characteristics.
Isn't better to say 'Apart from our previous study,'
Does 'is concerned' sound ok?

11) This justifies the claim to novelty.
Isn't better to say 'this justifies the novelty claim'?