If Nugent (why everyone calls him "von Westmeath"? Nugent was a professional soldier born in Ireland and added "von Westmeath" to his name when he was ennobled, from the name of the county where he was born) is routed, Austria looses the last field army they have in Italy, and not only Veneto (which is effectively already gone) but also Friuli (where in any case both Udine and the fortress of Palmanova are in insurgent hands) and his retreat across hostile territory would be a nightmare. This is only the beginning, though: Bohemia and Wien are already out of imperial control, and another defeat would certainly push the Hungarians in full revolt. Is this a reasonable bet for Nugent to take? OTOH, if he takes a more cautious approach and retreats behind the Piave river (which is a much better defensive line), he keeps his army "in being", not just as an useful counter for the negotiations but also to be used to reverse the situation in Austria proper and Bohemia, if needed, or to join the Croatians and invade Hungary if they go into full revolt.
But the point, as I said before, is that he doesn't necessarily has to fight, or maybe he'll be needed to fight elsewhere. I think your numbers may be a little optimistic: Nugent took with him 12,000 men, and even accepting that he found 35,000 men in Veneto (desertions in the Austrian army would have been certainly higher ITTL, in particular in the Hungarian regiments) his supply train cannot be very full, and his supply convoys will be harassed by insurgents, forcing him to escort them with whatever cavalry he has left.
The Brenta is not a big river, although in April the snow melt will increase the flow. However, there would be nothing more stupid for the Piedmontese to do to try to force the river at the bridge of Fontaniva, where the main east-west road crosses the Brenta. What is CA trying to do? Snatch defeat from the jaws of victory? The Brenta can be crossed at other points too, probably the best choice is at Bassano del Grappa, some 20 km north of Fontaniva. CA should keep some pressure and build up field fortifications at Fontaniva and send the bulk of the troops to cross at Bassano. There is an obvious advantage in having a numerical superiority on the enemy.
This assuming that CA needs a decisive victory, which effectively he doesn't at this point in time. Spending the summer organizing Lombardy and training additional troops, and at the same time carrying on negotiations via UK and France.
The only aggressive move he should do, it is to send the Sardinian fleet to the Adriatic. It would be the right move to give confidence to Venice, and also to carry out commerce raiding and such. The Austrian fleet (which was not much to start with) has been weakened by the mutiny in Pola, both by the loss of ships and trained sailors and will be in trouble to cover the Dalmatian coast.