Which of these monarchies will still exist in 2050?

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This is all very speculative; even the most seemingly stable of monarchies can fall.

I would say the safest monarch may be Japan's, being a stable, yet conservative country. Andorra, Liechtenstein and Luxembourg are probably safe as well.

Beyond that, it's hard to say. Many Muslim majority countries - the Gulf, Jordan, Morocco - are vulnerable to both Al-Quada and Islamist groups. We'll probably see them slip from absolute to semi-constitutional (or even constitutional, in the case of Morocco). Jordan's monarchy is in most danger - it isn't particularly liked at present, and the country sits in a vulnerable area and lacks the oil wealth of its neighbours to the South. And of course, who knows what will happen once the Thai king dies?

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If Belgium was dissolved, would its current monarch reign over both Flanders and Walloon? (Like the Scots arrangement)

The general trend regarding monarchy has been towards abolition  - and once they're abolished, they're pretty hard to get back. The monarchy that really needs to end btw is Swaziland.

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Bump to rethink my answers:

I think the Gulf monarchies are the most endangered. The Sauds have built their house on sand, and a lot of cracks are showing: the economic situation is dreadful and they're burning through foreign reserves, the war on Yemen is painfully unpopular, the West seem to be taking a more ambiguous stance towards the conflict with Iran and are under increasing pressure to distance themselves from their ally, the radical Islamists loathe them, they upper ranks are geriatric and the lower ranks poisonous and ambitious. It's not going to last. (Be interesting to see if the name changes, though). A Saudi fall would cause the collapse of the Bahraini monarchy almost immediately, and probably Jordan. Morocco is also quite vulnerable, especially if the "tame Islamists" of the PJd lose their popularity; as are the smaller states of UAE, Kuwait and Qatar, none of which seem like entirely stable situations. Perhaps The Omani monarchy is best suited for a constitutional monarchy  - Oman often goes its own way - but I couldn't count it out.

Of the Europeans, well, these monarchies have typically been through some pretty bad experiences and survived, so it would take some effort to dislodge them. Felipe VI seems to have saved the Bourbon's bacon, and he may be helped by more republican areas leaving. The Belgian monarchy may come under some pretty hair-raising struggles as the country may split in twain, but it held together after WW2 and Leopald's Nazi collaboration so im sure they'll at least rule a rump Flanders. The Dutch, Scandinavian and micro state monarchs are safe, assuming no serious scandals some. (Though I do note the Norwegians seem to in a scandal atm)

The Asian monarchs are probably also safe as the Scandanavians, with the exception of the SE Asian ones. Thailand's King is popular, but seeing as the army are intent on the idiot Crown Prince following him I doubt that will last. In a country as unstable as Thailand, that could be a mistake. Malaysia is an interesting case. The current king is mostly beloved by ethnic Malays, but if the opposing coalition, which has a lot of Chinese and Indians, they may be more sceptical of laws protecting the Sultans (even if they promise they are dedicated monarchists)

I think the Commonwelath realms will also drift away - Barbados, Jamaica, Australia, Papue New Guinea and New Zealand.

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The Republicans best time to strike is the period of time Charles is on the throne.

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I don't think many leaders (Kims aside) will do a Bakassa and self-style themselves as monarchs anymore. Most modern dictators these days are too technocratic for that sort of stuff. I mean even Gadaffi didn't try that!

I suppose the hard thing about predicting this is it is very dependent on the volatile actions of individuals. After all, if it wasn't for Prince Dipendra going Columbine on his family, the Nepalese monarchy would probably be muddling along in power by now.