The answers to your questions a whole lot of maybes and more information is needed to answer them accurately...
What type of keys and what DNs are the "Line 1" and "Line 2" keys? Are they the same on both sets? The most practical way to do this is to make appearances on both phones of the same number that are Key System keys (Multicall won't work).
Voicemail can generally be remotely accessed, the exact procedure will vary depending on your voicemail system. Assuming that it is embedded VM you can: 1) Point a DID number to the VM hunt group, 2) Call your own DID and when the VM answers press "*" and it will prompt you for your password, or 3) Call the main number and ask the receptionist to transfer you to VM and when it answers enter *<ext #> and follow prompts. There are probably other ways, but these are the most common.
You can forward your phone to your cell, assuming you have the correct COS options on your station and trunks, and the correct COR on the trunks, you just enter the Call Forward feature code and dial the external number.