At the beginning of Third Age after the Batttle of the Last Alliance Mordor was deserted and watched by men of Gondor to prevent evil creatures returing into Mordor. Gondor built the Towers of Teeth and the Tower of Cirith Ungol to that purpose.
The Silmarillion - Of the Rings of Power and the Third Age said:
The Númenóreans indeed set a guard upon the land of Mordor, but none dared dwell there because of the terror of the memory of Sauron,...
When the Great Plague came over Gondor Mordor was left unguarded (T.A. 1640) and some centuries later the Nazgul came to Mordor (T.A. 1980).
The Silmarillion - Of the Rings of Power and the Third Age said:
And in the days of Telemnar, the third and twentieth of the line of Meneldil, a plague came upon dark winds out of the east, and it smote the King and his children, and many of the people of Gondor perished. Then the forts on the borders of Mordor were deserted, and Minas Ithil was emptied of its people; and evil entered again into the Black Land secretly, and the ashes of Gorgoroth were stirred as by a cold wind, for dark shapes gathered there. It is said that these were indeed the Úlairi, whom Sauron called the Nazgûl, the Nine Ringwraiths that had long remained hidden, but returned now to prepare the ways of their Master, for he had begun to grow again.
Some 20 years later in T.A. 2000 the Nazgul came out of Mordor and took the partially deserted Minas Ithil. Except of this attack no major attack came out of Mordor in all the Third Age before the War of the Ring. Apparently there were some orcs in Mordor as in T.A. 2901 "Uruks of Mordor" attacked Ithilien but that was a rather small attack. All major attacks on Gondor came from north (Wainrider/Balchoth) or south (Harad) of Mordor
Sauron himself returned to Mordor only 67 years before the War of the Ring in T.A. 2951 when he started to rebuild Barad-dûr (RotK - Appendix B). Before that time I don't believe in much infrastructure in Mordor. There might have been some small orcish settlements but not much more.
When Sauron returned he quickly created a lot of warfare infrastructure at the Plains of Gorgoroth:
RotK - The Land of Shadow said:
Here in the northward regions were the mines and forges, and the musterings of long-planned war; and here the Dark Power, moving its armies like pieces on the board, was gathering them together.
I understand this as Gorgoroth was solely for military infrastructure. And there were a lot:
RotK - The Land of Shadow said:
As far as their eyes could reach, along the skirts of the Morgai and away southward, there were camps, some of tents, some ordered like small towns. One of the largest of these was right below them.
Mount Doom's activity seems to be connected to Sauron's presence in Mordor. I can't find any reports about any eruptions in Third Age before T.A. 2954, only three years after Sauron returned. But even at active times of Mount Doom like during the War of the Ring Mordor was NOT sheltered from the sun for most of the time. The Darkness of Mordor during the War of the Ring holds only for a few days. When Sam and Frodo come to Gorgoroth they were able to see quite far:
RotK - Mount Doom said:
Sam looked up out of the hollow. The land all about was dreary, flat and drab-hued. On the roads nearby nothing was moving now; but Sam feared the watchful eyes on the wall of the Isenmouth, no more than a furlong away northward. South-eastward, far off like a dark standing shadow, loomed the Mountain. … A few miles to the north-east the foothills of the Ashen Mountains stood like sombre grey ghosts, behind which the misty northern heights rose like a line of distant cloud hardly darker than the lowering sky.
Especially the last sentence sound to me like clear though very hazy weather. By far not all orcs got weak in sunlight. I think that's mainly a nature of the mountain orcs of the North.
My Conclusion:
I would suggest to build a lot of camps and primitive towns in Gorgoroth but nothing old or well build. Also I would build a lot of underground structures in Udun mainly as they are mentionend in The Two Towers.