Is it permissible to take the delayed payment of the deceased uncle's pension, and are the fees of the متابع (follow-up person/agent) permissible, and is it permissible not to inform the grandmother of this?

If the retirement pension was deducted from the deceased's salary, then it is a right for the heirs, each according to their share. However, if it is a grant from the state, then its conditions must be referred to in order to determine the beneficiaries and the amount of entitlement.

It is not permissible to dispose of another person's share of the pension or to take payment for work without their permission. If they permit and agree to the payment, there is no objection. If they do not agree after permission for the work and the custom is that the work is for payment, then the disposer may take a customary payment ( اجر المثل).

If the grandmother has aged and her mind has deteriorated, and she is no longer able to manage her money properly, then her case must be brought before the Sharia court to have her interdicted and a guardian appointed for her money. The guardian's actions are restricted by what is in her best interest. If the guardian approves your action, and it is in the grandmother's best interest, and the payment you received is a customary payment (اجر المثل) or less, then there is no objection. It is not permissible for an interdicted person to donate any of her money.

If you are the grandmother's guardian, your actions must be restricted by her financial best interest. Jurists differ on the permissibility of a guardian eating from the money of the ward. Some permit it out of necessity and for a customary payment (اجر المثل) or sufficiency, while others prohibit it for those who are not in need.

You are not permitted to unilaterally determine your payment; rather, this should be referred to custom or the judiciary.

However, if the grandmother's mind has not deteriorated, and she has authorized your actions and willingly given you payment and gifted you money, then there is no sharia objection to that.