Assume one may come and shorten your response time in advance. Keep originals accessible rather than shipped, hold certified copies and translations of anything already prepared, and record where each document came from so a replacement can be ordered quickly. Submissions go through the official portal following the instructions issued for Form I-140G.
Build a travel-proof document kit
Scan everything before packing, store the scans where two people in the household can reach them, and carry originals of civil documents in hand luggage rather than freight. Note the issuing authority and reference number for each certificate so a replacement can be requested from abroad. If a document takes weeks to obtain in your home country, get it before departure rather than discovering the delay while a request is pending.
Build the kit around what would be hardest to replace from abroad, since that is where a request causes real delay. Civil registry documents from a country the family has left, records held by a former employer, and anything requiring an apostille all belong in the first group. Obtain them before departure and keep the reference numbers, because ordering a replacement from another continent is slow and sometimes requires a local representative.
Note also that some documents expire for these purposes, so record the date each was issued and check with counsel which ones have a shelf life before assembling a file too far in advance.