Chaos:
There is no good reason for a draw rule to be excessively complex or subjective. All rules must be objective in the ability to enforce them.
I assert that ratings differences can have no place in any rule other than how ratings change with a win, draw or loss.
There is no stalemate in any variation of Espionage since the inability to make a legal move is a loss (if I understand it correctly). There are two other ways to lose. They are losing the HQ to enemy capture and losing all movable pieces to enemy capture.

The situation of various pieces that can avoid capture need not be addressed subjectively with their relative positions since it can be addressed objectively with the number of moves made without a capture being made by either player. When that threshold is met, either player can call it a draw without agreement by the opponent. If it were to require agreement, it would not be a rule and could not be enforced.
Regardless of any objections to such a rule, it would serve to keep the game moving at a more reasonable pace than would be the case without it.

If the move threshold without a capture is met, and a piece is subsequently captured prior to either player calling a draw, the threshold count is reset on the move the capture was made.

I suggest there are two things to discuss for draw rules. One is the number of moves for the threshold for each variation. The other is a three-time repetition of positions draw (with it being the same player's turn to move). Such a rule may be difficult with unidentified pieces, but possible since there is tracking with the move list. This might have the additional benefit of compelling development early in a game.