Is it OK to ask a person observing Shloshim to be the Sandak? Is he allowed to accept? If he accepts, is he allowed to remain beyond the actual Brit ceremony (for the Seudah)? If he is allowed to participate in the Seudah, is his participation limited in any way (i.e. may he sit with the family, may he lead / answer the invitation to Birkat Hamazon, etc.)?

It is permissible for a mourner to be the Sandak. A mourner for his father or mother who is Sandak is allowed to participate in the Seudah only if the brit is after the Shloshim. In your case, the Sandak would not be permitted to participate in the Seudah (Gesher Hachayyim I 21, 8,10; Pnei Baruch 20 14).