A group of fictional characters can solve problems without making one of the characters out to be the second coming of Christ.The book literally starts with the Imperials attacking Grantville and being saved by Gustavus cavalry and like you say its hard to write a story with many characters without many of them being genereic not to mention the bewildering number of authors involved in the series.
Marysueing is IMHO is due to the ISOT genre, wouldnt be much of a book if all are hit by plagues, lack of medicine, total breakdown of society in the uptime community as would be realistic
The event with Gustavus that you described is near the end of the book. The first fight in the book is against a group of rapists from an unknown army, so it's valid they fought against those people. They encountered several other military groups and were quick to make friends with one group that happened to be from Scotland without knowing which side they're on and what their principles are. They gleefully slaughtered most of the military groups they saw without knowing which side they're on. or what their principles are. Fortunately, they chose rightly, because everyone in the book has magic powers in judging people's character.
And Ryan, I didn't get the impression that the first book is against the German race. More accurately, the first book portrays Germany as being in a very sad situation that needs to be fixed.