I haven't seen the complete language but if you are all listed as grantors, all grantors may be giving up their rights permanently.
If it only listed your mother, she would be selling her rights and there is no need to mention you. The fact that you are mentioned as grantor leads me to believe that you need to get this in front of a lawyer, pronto.
"'..Grantor also grants sell and conveys to Grantee, its successor and assigns, by this deed ......(iii)all proceeds without limitation, all royalties, oil payments, gas payments,production payments, pooling payments, escheated funds, revenues payments, accounts,suspended funds, refunds, interest and overdue payments and others of value .... attributable to the Land before the date of this conveyance '"
The above, to me at least and with no knowledge but what you have given it could be construed that you are selling ALL royalty your father ever received. That essentially they hand your mom a check for the mineral rights but now they can demand all the money ever received from production from the land.
Read it this way....'..Grantor also grants sell and conveys to Grantee, its successor and assigns, by this deed ......(iii)all proceeds without limitation,.... attributable to the Land BEFORE the date of this conveyance '
To me, the above means you are not only selling the minerals, but also all the money that has ever been received from the minerals...for the purchase price. It sounds to me like they are trying to buy your minerals forever...with your own money. Or possibly you could wind up granting them your minerals and still OWE them money because the purchase check didn't cover it.
Once again, get this offer in front of a lawyer, PRONTO!
{.... attributable to the Land before the date of this conveyance '} Means all the money ever received. If it said AFTER the date of the conveyance, they would probably still be entitled to whatever was in suspense.
I think it possible they are really trying to stick it to you.