Both his first and third son died in 1520 as well, and if he died in late 1520 then it is before he got Hurrem pregnant with his fourth. That means his son Sehzade is the sole heir to the throne, and while he still needs to a regent the Ottoman government is stable.
Şehzade means prince, you mean Mustafa.
You're right though, for some reason I thought Murad lived longer. With that being said, Mehmed was born on 1521 October 31st, so Hürrem could already by pregnant if Suleiman dies in February. But alright, let's stick to Mustafa being the sole heir.
How would the regency look like? The Grand Vizier at the time was Piri Mehmed Pasha, who was rather competent (he played central role in the OTL conquests of Zemun, Belgrade and Rhodes), but there were numerous other figures at the Porte vying for power (Ahmed Pasha, Ibrahim Pasha, Mahidevran Hatun). All of them would try to influence the young Sultan, possibly recreating something similar to Louis II of Hungary's situation from a few years back.
Venice and the Knights of Rhodes rejoice!
Oh they certainly do!
Probably the Persians start making noises maybe attack,
The Safavids would have a prime opportunity to avenge Chaldiran. Could they reconquer Mosul though?
King Louis of Hungary doesn't die, so thats nice, no Mohacs makes things better.
Certainly, but even before that, Belgrade, Zemun and Sabach don't fall either, and neither do Orsova, Knin and Severin. Probably.
With the defences of the kingdom remaining more or less intact, Hungary would be in a better position compared to OTL. Beyond that however, I don't think the Hungarians would be able to really take advantage of the Ottoman situation.
The Ottoman-Hungarian truce was supposed to last until 1522 and I think it would be upheld. The Hungarian king was still underage and the treasury was too empty to wage a war. Louis II was rather hawkish against the Turks though, so I expect a war a bit later, probably during the temporary lull of the Italian Wars after the War of the League of Cognac. Until then, Louis would have time to sort out domestic problems, get the finances in order, prepare the army and maybe even produce an heir. How much of these could he get done and to what degree is anyone's guess.
Probably all the Balkan tributaries revolt, Serbs, Vlachs, maybe Bulgars.
I don't think any conquered land on the Balkans would revolt, but Wallachia and Moldavia would be able to shake off Ottoman influence for the time being.
Hungary was a barely functioning state at this point, with central government bankrupt and pretty much all the power being in the hands of the corrupt magnates. Their only real chance of survival is to be inherited by the Habsburg, who would have been able to modernize and maintain the southern border forts.
That's a bit extreme, imo. Magnate dominance was the fact of the day only because Louis II was still a child when his father first got incapacitated then died. Louis II was quite active in restoring royal authority: He reclaimed the possessions of the extinct Újlaki Family (which used to be one of the wealthiest, most influential families of Hungary) for the treasury, he took back royal estates pawned to Szapolyai and he also "renationalised" all mines which were previously pawned to the Fuggers. Louis II had a strong opposition, which made the management of the state challenging, but he was slowly and steadily clearing the obstacles from his way. Werbőczy has practically withdrawn from public life by 1526, Szapolyai was in a quasi-exile in Transylvania, George of Ansbach left Hungary. The only remaining great magnate at the court was Báthori.
