Thank you for your enquiry.
The issues you have raised are complicated and should really be addressed through instructions with solicitors and they charge circa £160 for a consultation but i will give you a broad answer to help you resolve any confusion.
Those who rely on the Exception under a family-based visa are usually given 10 years to settlement and this is particularly in cases where an applicant has an adverse immigration history. So if you have previously overstayed your visa or had an irregular immigration history, the appropriate route would be under the Exception. Your solicitor would have had access to all the information to decide what was the appropriate route to take. In any event there is nothing you can do about that to get ILR early.
If you have split up with your wife, your current visa as a spouse would no longer be available to you and you would be required to apply as a parent of a British child.
There are no limitations on how long your child can stay abroad as he is a British citizen. But if your child travels abroad and remains abroad, it would affect your visa as a Parent. Your child has to be living in the UK for your visa as a Parent to remain valid.
Your current situation is that you have split up, getting back with your wife for the purpose of securing immigration status is not allowed and should be avoided. The requirement of a family-based visa is that there exists a relationship that is genuine and subsisting. Therefore being married in itself with a marriage certificate does not satisfy the requirement if the relationship is not genuine and subsisting.
Getting a visa from an EU country does not in itself make you eligible to remain in the UK.
I would be grateful to be rated.
There is no guideline within the immigration rules on how long your child can stay abroad but a pattern of travelling abroad regularly may suggest that the child is not settled in the UK. The Home Office may look to where the child is registered in school as one indicator.