What is the odd little dot I see हिंदी instead of the न ligature on this spelling for हिन्दी ?
I've got as far as working out that dot is probably called "anuswaar"
Are these two versions pronounced the same way? One source sayz yes, another source says ं is a short version of न or म
Does anyone here read Urdu? am i guessing right that َن is न, َم is म, and ں is ं “anuswaar”?
As you've found, the dot is called 'anusvaar' or sometimes just 'bindu'(dot).
The anusvaar needs to be pronounced as a nasal consonant with the same place of articulation as the following letter. For example, in हिंदी, the letter after the anusvaar is द which is a dental consonant. So, the dot needs to be pronounced as न् (न without its associated vowel) which is the dental nasal. This is why हिंदी is interchangeable with हिन्दी.
Similarly, in लंबा (tall), the letter after the anusvaar is ब which is a labial consonant. So, the anusvaar is pronounced as म् (म without its associated vowel) which is the labial nasal and लंबा can be written लम्बा.
Of the five nasal consonants in Hindi, न, म and to a lesser extent ण are pretty commonly found even on their own in words. So, when the anuswaar stands for one of these, it is up to you whether you want to use the spelling with the anuswaar or the one with the conjunct consonant.
The other two nasals, ङ and ञ are quite rare in Hindi on their own. So, these are not written in the conjunct consonant form and the spelling with the anuswaar is always retained. For example, पंखा (fan) is never written as पङ्खा.
A related concept is the 'anunaasik' or 'chandrabindu' (moon-dot) ँ which stands for vowel nasalisation. For example in माँ (mother), the chandrabindu is telling us that the ā vowel needs to be nasalised. However, in words where there is no space above the top-line of the letter for the chandrabindu due to other diacritics, the bindu is used in its place. For example, in मैं (I), the dot is the anunaasik not the anusvaar.
I can't read Urdu but as far as I know, Urdu does not use the anusvaar and the nasals ङ, ञ, ण, न are all written the same. The other nasal letter in Urdu which you've mentioned may correspond to the chandrabindu of Hindi. I'm not sure of this though.