This is a very exciting project, great choice! I'd like to help you, but before we begin I'd like to know the exact scope of what you are going to emulating. Let me tell you several reasons why the scope is very important.

1. When the negotiations started, there was no precendence, therefore its protocols evolved over time.
2. The tides of the Thirty Years' War shifted significantly during the time it was being negotiated.
3. There was no one HRE party, as much as there were envoys representing their monarchs and sovereigns. To be more precise, member states of the HRE had been empowered to conduct their own foreign policy since 1644 as Ferdinand III had promulgated ius belli ac pacis.

Therefore, it is very important to determine the time period you want to study. Further, as the HRE as a whole was not a present entity, I shall take your question to mean, what was Volmer's intention going into Westphalia. Volmar was representing Ferdinand III, who held many titles, one of which was Holy Roman Emperor. However, Volmar was not THE representative during the critical part of the negotiation: he was second behind Count Trauttsmanndorf, himself the prime minister of Ferdinand III.

Before we get into the detail of how the conditions shifted and impacted negotiations, it is fairly common to partition the negotiations into three phases:

• The first phase from 1643-1645, where procedures were negotiated.
• The second phase from November 1645 when Count Trauttsmanndorf arrived, until June 1647 when he left. This was when most disputes were settled.
• The final phase when the settlements of the disputes were drafted into treaties.

As far as I know, Volmar was present for all these phases, and effectively took over when Trauttsmanndorf left and shortly died due to illness.

After all that discussion, what was Ferdinand III's consideration in Westphalia? There are several.

• Control the damage from the decline of Spain. Over time, the Thirty Years' War saw increasing interference from Spain (read: increasing dependence of the Catholic party to assistance from Spain), which then declined following internal (revolts of Catalan, Portugal) and external (rise of France, losses in the Low Countries) factors.
• Ensure survival of his family as Holy Roman Emperors, in the face of continuing fragmentation in the HRE and failure to unite the HRE under Habsburg leadership.
• Religion and state, as many in the Catholic party still believed that the Peace of Augsburg was temporary. In particular, the Papal party tended to want to reject any compromise on religion. Ferdinand III needs support from the Papal party, but is limited in how he can negotiate.
• Unity among the Catholics. Parker sums this up sharply: although the emperor was on the side of of the Catholics, not all Catholics were on the side of the emperor.
• Play France and Sweden off each other on the Protestant side. The two major backers had their own views. Sweden wanted a lasting domain in northern Germany. France wanted modest territorial gains in return to continuing weakness of the HRE. In the waning days, France was facing its own revolts (the Frondes) and felt that time was not on their side. But French eagerness to compromise worried the Swedes, who began their own sprint to peace.

You should consider reading Parker's The Thirty Years War and Croxton's The Last Christian Peace to best prepare!