1. Use the letter with reference number from Niagara Fall office to your lacal office for SSN printout before receiving your SSN Card (Mine was processed for one week for the verification, and needs another week or two for the SSN Card). HR only needs SSN number for I-9 Form. However, technically you can start working without SSN number or SSN card. BTW, any number you get will the unique one assigned to you. There is nothing called temorary number for SSN.
2. For I-94, if you drive through POEs, you don't need to yield - just show officer I-94 and say you want to keep it for I-9 form. I got this information through talking to border officers. They also don't want the trouble to take off old I-94 and issue another new one. However, if you enter US by air, you may need to yield.