I heard as long as someone continued to maintain financial ties with US, they can use SB-1 to return even after one year has passed since stepping outside the country. However, if an unexpired B2 visa obtained prior to getting the green card already exists, can that be used in lieu of SB-1, to re-enter?

On the other hand, if SB-1 is mandatory, is that sufficient to re-enter as long as green card has not expired or does one need to apply for additional visas or go through additional procedures before permitted back into the country?

On a related note, what are some of the advantages of surrendering the green card using I-407, if flying back to US continues to be a challenge due to the current pandemic? The green card was obtained 2 years ago.

To get an SB-1 returning resident visa, you need to prove to the consulate that you couldn't return to the US any earlier due to circumstances beyond your control. It is not as simple as "maintaining financial ties with the US".

It may be possible for you to try to return to the US after an absence of one year with just your green card, without a re-entry permit or SB-1 visa. There is a chance the officer will let you in if you can convince him that you did not abandon residence.

Trying to use a nonimmigrant B2 visa to enter the US would raise serious questions about why you are trying to enter as a nonimmigrant if you are a permanent resident. It might be taken as a sign that you are intending to give up your permanent residence. You should not do this if you want to keep your green card.

Officially you must have the SB1 to return to the US after being outside the US for over one year. As noted by the other poster, if you attempt to enter without one, you would be pulled aside and interviewed and they may allow you to enter but it is more likely you would be forced to return to apply for proper paper work. If you knew of the extended absence prior to leaving, you should have applied then. But yes extended absence can be a reason to revoke the green card, which should have been explained to you when you got the card