Looking at XC Weather and having been onboard an H28 a little bit, Monday looks ok, but likely to be very wet if setting off from Chichester as the sea will be just off the nose and that's wet in a 28.
Tuesday looks ok for an early morning dash, but need to be away just after lunch for a dash back.
Best thing to do is poke the nose out and follow the Island Shore to Ventor, see how it feels once out of the lee of IOW, most importantly don't be scared to turn back, plenty of nice lunches to be had hereabouts and a bad day on the water is always better than a good day in the office. (Unless you sink/catch fire!!)
Monday late pm looks fine. The trip back on the Tuesday is the one that the forecasts disagree on whether the change was going to come at midday or 7am. As you say, the answer is probably to poke our nose out into the channel on Monday pm and suck it and see....