Neither the placement of Agar / Udul nor Ishmalog is clear
This I would contend. Here's a note Tolkien wrote in the early 1960s, with the relevant passage in bold:
"Tal-Elmar.
Beginnings of a tale that sees the Númenóreans from the point of view of the Wild Men. It was begun without much consideration of geography (or the situation as envisaged in The Lord of the Rings). But either it must remain as a separate tale only vaguely linked with the developed Lord of the Rings history, or –
and I think so – it must recount the coming of the Númenóreans (Elf-friends) before the Downfall, and represent their choice of permanent havens.
So the geography must be made to fit that of the mouths of Anduin and the Langstrand."
Furthermore, a quote from Michael Martinez on the subject:
"Since Tolkien thought the story should be set in the Langstrand, it seems logical to infer that he might have identified the green hills of Agar with the Pinnath Gelin (“green hills”) of Gondor in the later Third Age."
So Agar was probably located south of the Pinnath Gelin - and certainly not in Drúwaith Iaur.
I was only thinking it could be a pretext to build a nice valley...
You'll perhaps be interested to know that Ishmalog is (perhaps) already made.
Tolkien implied that the "North King" mentioned in the battle was Sauron, and that the people of Agar served Sauron in this particular defensive war. The "Fell Folk" whom they fought were described as being tall and fair-skinned, and thus were probably related to the Marachians or Beorians of the First Age and came from Rhovanion (i.e. ancestors of the Northmen and Éothéod of later years).
If these assumptions are correct, then the southern Vales of Anduin would be the only route by which Men of Rhovanion could easily reach the area that the people of Agar inhabited. The vales of Anduin would also have been a strategic region between Mordor and the eastern lands requiring defence against incursions from the north. It therefore seems likely to me that Ishmalog is the valley between the Ered Nimrais and Ephel Dúath which would later became the site of Minas Tirith and Osgiliath.