Is wishing for the return of the deceased and paying money for that considered shirk (polytheism), and is it obligatory to pronounce the two testimonies of faith (Shahadatayn) and seek forgiveness when feeling embarrassed by such talk?

The answer to your question is in two points:

Firstly: The phrase: "If Abd al-Ilah were to return to us, we would give all that we own" does not contain anything that necessitates doubt about disbelief or anything close to it, nor does it contradict the fact that the matter is in Allah Almighty's hand and will. Rather, it appears that it is not objectionable or sinful at all, and there is no impediment to using "if" here.

Secondly: Uttering the two testimonies of faith when in doubt about committing an act of disbelief, as a way of returning to Islam, is not commendable or enjoined, especially for those who suffer from obsessive doubts.

We advise the questioner to turn away from such thoughts that open the door to obsessive doubts, and to focus on what benefits him.