My biggest worry is that there maybe be passages that are unplayable, such as consecutive notes that are too difficult for a non-virtuosic string section. There are key signature changes about every 4 measures.
Frequently-changing key signatures won't bother anyone who is accustomed to playing music written since 1900. It sounds like the underlying pulse remains the same, which makes things easier. Rapid consecutive notes on a single pitch (like at the beginning of your piece) are not that hard for strings. Rapid consecutive notes on moving pitches can be difficult depending on the intervals. If you're concerned, check with a string player. Maybe post a part on the strings forum here.
The horn and oboe were not intended as solo parts but as part of the ensemble.
The term "Concerto Grosso" means a piece for a small group of soloists surrounded by an orchestra, taking turns with an orchestra. "Concerto" (noun) or "Concertante" (adjective) means one or more featured soloists accompanied by an orchestra. Usually something called "concerto" is more of a starring role for the soloist, whereas "concertante" implies more integration between soloist and ensemble. Generally speaking.
In this day and age composers can title things however they want, so there's nothing wrong with the title, but calling this a "Concerto Grosso" is a bit like writing a piece for strings and then calling it "Piano Sonata."
Pieces for small orchestra could be called Sinfonias/Symphonies, Overtures, Suites, Serenades, some creative title like "Fanfare upon the Hills after Handel," the precisely descriptive "Piece for Small Orchestra," or really anything you can think of. In the baroque period "suite" meant a specified series of dance rhythms, but that isn't the case anymore.