Yes, this… 
So, thinking about it, does that mean you’d also hear byddet ti’n 'neud o? And do any of the alternatives sound posher or more formal or more old-fashioned, or whatever? (I’m sure it’s easier just to accept that they all exist and not worry about it – which is, in fact, exactly what I’ll do – but I’m wondering just now…)
In my experience, gwneud often gets shortened to 'neud. However, I don’t think saying gwneud is equivalent to say - cannot vs can’t. Just my experience though, please don’t take this as gospel 
There’s probably a lot of personal feel in there - I think I tend to notice when it becomes just ‘neud’ and see it as very informal - but I’m sure there are plenty of times when I wouldn’t even really pick up on it… yes, you’d definitely hear ‘byddet ti’n neud o’…
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No, it stands for both ei and eu
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