It is my understanding that you are interested in knowing how this phrase fits into the whole of your text. In Spanish criminal procedure certain types of "autos" may be subject to two types of remedies: "recurso de reforma" and "recurso de apelación". When it is stated that a "recurso de apelación es subsidiario al de reforma" what is meant is that in order to file a "recurso de apelación" the appellant must have first sought redress via a "recurso de reforma." If then the appellant is still not satisfied with the outcome of the "recurso de reforma" he may subsequently file a "recurso de apelación". The difference between the two appeals is that a "recurso de reforma" is a petition to the same court that issued the initial "auto" to review or reconsider its decision (known as a "recurso no devolutivo"). In contrast, a "recurso de apelación" is what is actually understood in Anglo-American jurisdictions as an "appeal", that is, an petition to a higher instance to review a lower court's decision (called a "recurso devolutivo").

The translation I offer is admittedly long and drawn out, but I'm not sure there is a nice snappy way of expressing this in English.

To offer some observations on your proposed translation:

You indicate that there is a “pending resolution”—In my reading of the text there is no resolution pending: “Ese auto fue recurrido el XXX de 2010 por los querellantes, en reforma y subsidiariamente en apelación” simply states that the decision was first the subject of a “recurso de reforma” and that subsequently a “recurso de apelación” was filed. (This implies that the outcome of the "recurso de reforma" was not favorable to the parties and, for that reason, they subsequently appealed via a "recurso de apelación.)

“Ministerio Fiscal” refers to the “Public Prosecutor,” rather than to a “Public Defender," which (loosely) the counterpart of an "abogado de oficio" in Spain, that is, counsel for the accused.

Here are some definitions:

Recurso de Reforma--Medio de impugnación que se interpone ante el mismo Órgano judicial que ha dictado una resolución, que no sea sentencia, con el fin de que la modifique por ser perjudicial para la parte. (Diccionario Jurídico Colex, 2th ed., 2003)

Recurso de Apelación--Es un recurso ordinario y devolutivo que procede contra las resolutiones que indiquen las leyes procesales, y en cuya virtud un nuevo tribunal, unipersonal o colegiado, distinto y superior del primero adquiere competencia para que se vuelva a conocer de las pretensiones de las partes. (Diccionario Jurídico, Aranzadi, 3rd ed., 2004.)

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Note added at 11 hrs (2011-04-03 07:00:23 GMT)
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To add a comment to mediamatrix's observation:

When it is indicated that a "recurso de apelación es subsidiario al de reforma" this does indeed imply that both appeals can be filed in the same document ("escrito"). This procedure is provided for in Article 222 of the Spanish Criminal Procedure Act which states that "El recurso de apelación no podrá interponerse sino después de haberse ejercitado el de reforma; pero podrán interponerse ambos en un mismo escrito, en cuyo caso el de apelación se propondrá subsidiariamente, por si fuere desestimado el de reforma."

A typical writ of this type would be worded as follows:

"...Que por medio del presente escrito, dentro del plazo legalmente conferido al efecto, interpongo RECURSO DE REFORMA Y SUBSIDIARIO DE APELACIÓN contra el Auto de fecha......, de acuerdo con lo establecido en el art. 216 y siguientes de la Ley de Enjuiciamiento Criminal, con base las siguientes alegaciones:..."