I agree that all the modern straight Kattara probobly do have locally made blades. All the modern ones that I've seen are frankly 'only' fit for dancing with
But I've seen older ones with good double edged blades clearly of the same genre of trade blades exported en-masse from Europe, oft copied locally and seen in such varied incarnations as Kaskara, Mandinko swords, etc, etc...
Hold on, didn't I show you a short tang trade blade of the type commonly seen in Kaskara mounted up as a Kattara?
Anyway, the Shamshir.
What you seem to have above are two 'fairly standard' form Shamshir exported widely and in this incarnation re-dressed with some locally made mounts.
The form hasn't been altered a breath and if it wasn't for the close-up pictures you could easily not even notice that these have had a holiday in Oman.
I'm suprised that you're not going to source some plain or tatty shamshir and have your silverworkers redress them in Omani style?
As to 'who' added the Omani mounts to the originals?
These swords (and related types) were widely admired across half the world. Given the time you could probobly find dozens of retro-fitted and locally embellished Shamshir from as many different countries.
I would assume that these were simply imported of gifted swords given a slightly local flavour to 'Omanicise' them.
As to them being rare?
I would conjecture that many might not have been modified at all or only lightly re-dressed, so would only be distinguishable by knowledge of their actual provenance or possibly just by their scabbard?
Once removed from their direct history or parted from their Omanicised scabbard their 'connection' to Oman is lost.
Which is why the two complete examples that you show are so interesting.
There is no reason why Shamshir might not have been popular among certain wealthier "Omani" in times past and the majority might not have been modified at all.
So like many of our swords, their 'history' is lost over time.