In 2018 I crossed the Po Valley in the second half of September. I had no real issues with mosquito's or heat. The Po Valley was much better than I expected. Quiet lanes, tracks and country lanes, crossing a lot of small towns. I think April will be fine as well, as the real hot and mozzie months are June, July and August. Apart from this, a camino is a walk from A to B (Rome), not a bus hopping tour. Some suburbs, busy highways, and/or industrial estates are part of it. If you walk the whole Via , you will discover that those not so nice parts are only a tiny bit of the whole footpath.
Regarding the scenic part thru the Alps. I started from Gran San Bernardo and it was a great experience with beautiful views. However, if you really want rugged mountain scenery in France or Italy, don't walk the VF, but try Tour du Mont Blanc, Tour du Queyras or Valle Varaita Trekking. The VF association succeeded to create a very nice walking path from A to B, but you cannot compare it with scenic long distance trails thru wild nature.