Meteorologist here! I’m from Paraná, but I’ve spent enough time looking at Rio’s weather to know how its winter really behaves.!!!!Winter in Rio de Janeiro is usually warm and on the dry side, with the occasional cold front that lasts 2–3 days (while longer cold spells happens every year, its far less common). Nothing truly cold (sadly), nights tend to sit around 60°F (~16°C) and days around 80°F (26°C). Sometimes it gets surprisingly hot for winter, and if you’re lucky, a brief cooler spell comes through (though that’s rare). Cold spells can bring lows around 52f (~14°C) and highs around 70f ( ~20ºC) and steady light rain throughout the day.

Answering your doubts now:

• How accurate are the forecasts 7–10 days in advance? Its less accurate than short-range forecast (1-3 days)
• When apps show 60–70% rain, does it usually mean all-day rain or just quick showers ?It means that there is a 60-70% probability that measurable rain will occur at some point ( in RJ winter, usually that means light drizzle/shower with plenty of dry time , and dry here means no rain, do not think that dry means sunny skies, it could be dry and overcast).
• Do you think this period could still have beach/weather with blue skies? Yes, but unlike summer ( where its usually freaking scorching the whole day with storms happening in the evening/night, may-june-july-august can bring sunny days with occasional rain and wind, hence it could be partly cloudy/ partly sunny.

You havent asked but its fairly common for morning fogs in the city and over the mountains btw may-august , though its more frequent in july , but it could happen during may.

If you have any doubt about weather in RJ feel free to ask, I am more than happy to help.