OED

At the beginning of reference 1, I used the phrase 'illusions, not to say delusions' without much thought, beyond that the doubling up sounded well. This morning I woke to a concern that I was not very clear about the difference between the two words, so the second job after making the tea was to visit OED in the extension bookcase.

It turns out that both words are old, dating in English from the 13th century, and both are derived from the Latin 'ludere', to play.

The word before illusion is the hyphenated ill-use, but that seems to be a coincidence, with there being no link at the time the words emerged, although there might be a touch of conflation now.

Illusion is a noun, earning itself just about a column, and derived from the obsolete verb illude, to mock, deride, trick, deceive. Often used in the passive form 'I was illuded about something by something or someone'. Also to evade or elude. This last a corrupted derivation from exclude and include?

With illusion being the result of the illuding. But quite quickly the use slips away from its verbal origins and stands alone, with an illusion being something which deceives or deludes, which appears to be something which it is not. We lose the active agent.

In the case of delusion, the neighbouring irrelevant word is deluge, with delumbate (to lame by bashing the loin) and delungdung (an animal of Java) in between.

Otherwise rather like delusion, but with the difference that the original verb delude is still with us. An illusion just is, whereas a delusion usually involves an agent, someone who want to delude or otherwise cheat us. And with the option that, in the case of mental illness or instability, the verb can be used reflexively, that agent might be oneself. One might delude oneself for one reason or another.

So I think both the meaning of the two words and their order at reference 1 is reasonable. Dulude amplifies illude in a reasonable way.

And I dare say a linguist or a linguistician might be able to dilate at greater length on the use of 'i' flavoured prefixes and 'd' flavoured prefixes on words of Latin origin.

PS: quite wrong regarding elude and exclude. Elude and elusion is just another pair of words on the same lines as illude and illusion, from the Latin ludere. While exclude and include are from the Latin 'claudere', to shut, shut up or shut in. As with a door.

Reference 1: http://psmv3.blogspot.com/2018/08/the-myth-of-unconscious.html.