hi there,

welcome to qatar.

1)You will not be able to bring your wife on family visit until your RP is not over. Wait till your RP is over.

2)after that you can apply immedietly for family visit and you can get it in even one day. keep your documents ready.

marriage certificate
passport copies
birth certificates for your kids

for visit visa the attestation from your country's foreign affairs is not require but yes it is required if you want to convert into the resident permit for your wife and kids. so make sure you have it handy.

3) Family visit is extendable upto 6 months. once the family is here they are required to go through a medical upon its confirmation only the visa could be extended for the said period. QAR 200/month is charged for the visa and QAR 100 for the medical (one time).
4) Yes you can change the family visit to RP for your family.

hope this would suffice your doubts.