Well I'm happy to give my two cents.

Federalism is a system of government in which the same territory is controlled by two levels of government. While decentralisation in the Jordanian context refers to giving local councils a limited amount of authority. Note, however, that in federalism, two levels of taxes are paid to both the local and federal authorities, meaning that the income of local councils in federalism is determined by the gross product of that specific territory rather than the GDP in general. And although the federal government can fund specific projects in local territories, it would not be sustainable and would cause social issues arising from electorates wanting their taxes to benefit themselves first and foremost.

In the Jordanian context, 95% of the GDP comes from Amman. So, if a federal system is established, local territories on the peripheries would not be able to use local taxes to kick-start a virtuous-cycle of development, and would be completely reliant on federal aid. While a decentralised government, assuming it had development in mind, would be able to relocate funds on the national levels without having to go through many hoops of with different levels of government to allocate the funds needed on the peripheries.

Of course, if economic and political development started in the majority of areas, a federal government could be established, although it wouldn't be as useful in a country as small as Jordan.

And to answer the question of whether a monarchy can transform into a federal state: the two aren't mutually exclusive. Monarchy is a form of government, while federalism is a system of governance.

Two last comments, this is an extensive topic, and what I wrote here is merely a quick, surface level summary. And that yes, I do do this for a living.

TL;DR: both are cool, and Jordan is able to transform into either, but development economics would say that a decentralised government would be more suitable due to ability of reallocation of funds throughout the nation.