Salamun Alaikum,
'Abad' means to remain or stay. It does not always imply forever but can also imply a very long duration of time. The same is true for 'khalidun' which I discussed in the article you mention. So these verses cannot be read in isolation.
The best way to analyse this is to find verses where both hell and heaven are contrasted together.
In the following verse both heaven and hell are contrasted. But here there is a crucial difference.
In verse 64:9, when the dwellers of Janat are mentioned, the Quran says "khalidina"
with the 'abadan' to emhasise an eternity. In the next verse 64:10, when it comes to the companions of the Fire, it says "khalidina"
without the 'abad' implying a limitation in the duration of time.
So there is a marked difference when the two are contrasted together.
Now if we read 78:21-3
078.021-3“Truly Hell is as a place of ambush, For the transgressors a place of destination: They will dwell in it for ages (Arabic: Ahqab(a))”'Ahqaba' here does not mean forever. See how it is used in verse 18:60 to denote a long time / ages, but not forever.
Then contrast once again 11:106-7 (hell) with 11:108. In the second verse dealing with heaven, it says "Ata'an ghayra majdud" which means a gift without a break. This is missing from the verses that deals with hell, once again implying a limitation in the duration of time.
The article below provides a more detailed analysis.
IS THE PUNISHMENT OF HELL ETERNAL?http://quransmessage.com/articles/is%20the%20punishment%20of%20hell%20eternal%20FM3.htmI hope that helps, God willing.
Joseph.