What to do after I485 interview denial?

you can see if you have a valid reason to "re-open " or you will have to refile your case all over again

H4 Extension is in progress (I94 Expired sept2017), amendment question ?

you do not need an amendment for the H-4

What should i do? I received my H1 B visa extension approval on 04-16-2018 but both my visa and i-94 expired before 04-10-2018?

do you mean you received the notice on the 16th, or the "start date "is the 16th show an attorney the approval notice to advise you of your options

My wife who is a permanent resident will file I 30 for me. Will my own income be considered as affidavit of support?

Are you working " with permission"? meaning authorized by USCIS? if so, you can likely count it

Who is the principal and derivative applicant?

I agree with Mr Capriotti as usual .

Travel out of US while H1B extension and amendment is pending..have current valid stamped visa?

You should be fine , but i would check with the Lawyer who filed you H-1b extension BEFORE travelling

When changing a joint sponsor, do we need to state why we changed the joint sponsor?

Usually you do not need to give a reason, but check with a lawyer to make sure. more info is needed

Bringing my son to US

how did you get your green card> there is not enough info to answer this. you may be able to petitioner for your son, or he may be eligible to "...

Yes, you will have to refile all the petitions with all the fees all over again

After your priority date becomes current in F2A category, how long it typically takes to get GC ?

it depends if you are in the USA or out? It also depends if you have maintained lawful non-immigrant status i would contact a lawyer to go over...