1. Yes. It is a good idea. This way, the H4 visa stamping will be done in way less time vs if you apply for H4 extension in US itself. You being principal applicant, can take advantage of Automatic Revalidation Provision (ARP) and will not need a visa to re-enter US. Infant can re-enter on valid I-94. Unless you have appeared for an interview at US consulate either in Canada or Mexico, then APR will not be applicable and MUST re-enter US based on the outcome of the visa interview.
2. Yes
3. Yes.