Why didn't the tribes of Re'uven and Gad and 1/2 Menashe take up Moshe's suggestion?

Question

In Bemidbar 32:18 the tribes of Gad and Re'uven tell Moshe that they will not return back to their land until all the other tribes have inherited their land portion (after conquering the land and defeating their enemies.)

Moshe proposes in Bemidbar 32:22 that all they needed to do was to help their brothers in battle and defeat their enemies. Once the war was complete, they could return home. They need not wait until they inherited the land.

We see that in actuality, these 2 1/2 tribes waited until everyone inherited their portion before they returned (See Joshua 22) Why did these tribes not accept Moshe's offer?

Answer 1

Art Scroll on Matos 32:18 cites Haamek Hadavar that this was in order to allay suspicion that they would want to take advantage of their privileged position (having already received their land) to settle down earlier than the rest of the tribes.

Matos 32:22

you shall be freed [of your obligation] from the Lord and from Israel, and this land will become your heritage before the Lord.

shows that the land became theirs immediately after conquest, but they still stayed in order to show that they still belonged with everyone else.

As it says in Yoma 38a, that one must act in a way to avoid suspicion on the part of human beings and not just according to the letter of the law.

Answer 2

The original proposition made to Moshe by b'nei Gad and b'nei Reuvan was that they would not return until after helping conquer the land and all of the tribes were settled. Moshe responded that they must keep the tenay to help conquer the land. However, Moshe adds that they must also keep the neder they made not to return until after all the tribes are settled. See Bamidbar 32: 18, 22, 24. Especially Rashi on pasuk 24 d"h ve'hayotzei.

According to Rashi then, there was no possibility of only accepting Moshe's tenay because they had made a neder which they were required to uphold.