Very interesting insight.
OTOH, they could be reasonably successful: the Hapsburg forces and army of the Catholic League were at that time relatively small, the tactical advantages of the Western armies still were an issue of the future and Montecuccoli was just a boy so there would be a reasonably good chance if not of a smashing victory than at least in causing a lot of damage (and doing a lot of looting) which in a precarious pre-White Mountain situation could be enough to change the tide and force Ferdinand to acknowledge the loss of Bohemia.
Well, this still leaves 2 big issues:
1. Keeping Spanish Road open. Spanish troops still could march across Alsace (see the map). The problem was not in Germany but in Savoy and France: the
Treaty of Lyon (January 17, 1601) forced the Spanish Road to be reduced to a narrow valley and a bridge over the Rhône. This loss of territory made Spanish passage on the road dependent on the approval of France (which it refused in 1601 - 1602). "In 1609, Savoy expelled Spanish garrisons, followed by an alliance with France against Spain in 1610 and a dynastic war over possession of
Montferrat (1613–1617), settled by the
Peace of Asti. Savoy allowed a Spanish-Italian army to pass through the Spanish Road in 1620 but its anti-Spanish Treaty in 1622 ended Spanish travel on the Spanish Road forever." (
https://en.wikipedia.org/wiki/Spanish_Road#Use ) As I understand, the troops still could march through the mountain passes of Valteline/Graubunden and then to Hapsburg-held Tyrol (in OTL conflict over control of this area lasted from 1618 till 1639).
2. An issue of the Protestant vs. Catholic electors (especially when one of the Protestant electors holds 2 voting positions). This is serious but probably can be negotiated, even with the support of at least some Protestant princes (and electors) who are not very fond of Frederic and the whole Bohemian business (Saxony almost definitely, Brandenburg - possibly if there is a suitable carrot): prior to Ferdinand starting pushing them around, they are seemingly happy with the status quo. Say, something along the OTL lines: Frederic retains electoral status as King of Bohemia but his original electoral position is transferred to Bavaria (as did happen). The balance is retained, everybody is happy (for a while).