Sorry if I missed this earlier somewhere:

In this situation Player A is attacking Player B. Player A wins. Player B then casts random on Player A, apparently before Player A has taken his reward (life/object/gold).

Someone made the argument that you cant interrupt the 'space in time' between winning combat and gaining the reward…this same argument came up again like this:

Player A holds the Rod of Ruin . (The one that can remove up to two str/cft points from anyone, apparently at will, although the item does not state specifically when it can or cant be used). Player A attacks Player B. Player A looses, though upon realizing that the defender will likely take the Rod as a reward and use it it against them, Player A hastily uses the rod before Player B can claim it as a reward.

Legal? Is there a official combat sequence?

Bolithio said:

Official combat sequence is in the rulebook, page 11 (Battles between two characters). It's worth reading, because it contains the answer to the first of your questions. During battle and psychic combat, Spells can be cast before the attack roll is made, except when the Spell specifically states otherwise (e.g. Retribution Spell). After that, the combat must be completed before other Spells can be cast and influence the course of that combat, including Spells that are "cast at any time/as required".

As for effects like the Rod of Ruin, ditching Objects or turning them into gold with the Alchemist before reward is claimed, there's no official rule or FAQ available.

The simplest way to follow is as someone from your group already suggested: do not interrupt the sequence between "Compare Attack Scores" and "Claim Reward". However, if someone wants to ditch Objects, use them, or turn them into Gold before dice are rolled, he should be allowed to do that. Not afterwards.