In order of questioning…USCIS is working on electronic submission, but currently I-526 petitions with the supporting documentation is sent by post or FedEx. Second, USCIS policies provide for a 10-year renewal period for green cards. We comply with the regulations as they currently are. Third, there is no requirement to become a US Citizen, but you will be unable to take full advantage of citizenship (like voting) while still being required to maintain presence, pay taxes, etc. Finally, if an application for citizenship is denied, it can be appealed or refiled.
The filing of the I-526 can be done either electronically which may involve faster processing or by mail. The unconditional green card is issued for 10 years. The resident will then have to file an Application for a new updated green card for another 10 years. The USCIS in the renewal process will determine if there are any grounds of inadmissibility or removability for crimes committed by the investor/resident. A resident does not have to apply for Citizenship. If the resident applies for Citizenship and is denied the resident will still be able to retain the residency unless the denial is a ground for removal of the green card status.