Whether to say - 'Allah is one' is correct? Which of the vereses in Al-Quran substantiate the statement? I see Surah-Ikhlas (1121), wherein it is said - "Qul huwa Allahu ahadun". But not sure whether 'Ahadun' can be construed as 'one'?

Sorry, what exactly is your problem? You are bothered it didn't say 3? What can only be One? What can truly only ever be One is what is Allah, its a map to help you understand what Allah is perhaps, will you follow it?

The word " Unique" comes to mind. i.e There is none like Him.

GOD bless.
Peace.

There is not at all a physical object matter and living for the purpose of describing Him.

The word " Unique" comes to mind. i.e There is none like Him.

GOD bless.
Peace.

Salaam!

The last Ayah may help resolve the point.

You the Messenger tell the people "The absolute fact is that Allah is the Only and Alone; the Unique; the Solitary; the Absolute; the Perfect, the Exclusive.
Allah the Exalted is the Only Self Reliant; Transcendent, Absolutely Independent; One upon Whom all without exception are dependant;
He in Person never became the cause, and nor was He the effect.
And for Him there is always none ever to compare-exemplify; He is Unique-The Absolute".

This will also help
http//haqeeqat.org.pk/English%20Tafsir%20e%20Haqeeqat/00.Arabic%20Text/004.%20An%20Nisa/temp%2014%20%204.171a.gif
Undoubtedly Allah the Exalted is the Only-Solitary Iela'aha the Sustainer Lord.

Salaam!

I don't it is necessarily the case that " that which is countable is made up of some material and can be seen too"

Even so. If there is only one existence (this is the only logical possibility) and both God and existence encompass all things, then I don't think there is anything wrong with saying God is one. One God, one existence.

I think you need to say 'God is the One' in English or just 'the One'. This of course goes against grammatical rules as it is a name of a person with a definite article.

However, we can ask ourselves if to claim God to be a person and thus consequently making the One a viable choice. The One is more like talking about the entity 'God' as in His Oneness.

I use this definition for God relatively often and I believe the Arabic form is one of the most basic "names" of God. Since God does not have a single definite name you can simply call Him "the One". God allegedly said "I am that I am".

God bless you

Salaam!