Its really no big deal to cut in new soffet vents. a blind cut with a blind cut jig saw blade and your there. I would do the extra effort if it was my home. I would also make sure that no obstructions are hindering the circulation of the new and old soffet vents.
I would blow insulation under the decking for sure.
Blowing in insulation is great and easy, But one problem with this is the insulation comes out of the tube with such force that it acts like snow drifts and builds up against obstacles so be careful that you do not block the soffet vents and that you make sure there are NO buildups on the leading side of the 2x6 floor joists.
Remember the weight per square foot that you have and what your laying down. Do not become too heavy. Remember the setteling of this stuff.
Wear a mask and eye protection, dust becomes very thick.
Have a blower person and a loader in the hopper person. Set the machine on a dedicated electrical circut as it does eat amps.
Watch the can lights and exposed electrical connections if any.