Actual start date later than indicated on COS

[snickers85]

Hello,

I was advised over the phone that my visa had been 'approved'. As I believe the chances of it having been declined are slim (or it may have.... now that's a whole a lot of other issues...) I am anticipating its arrival in the next few days. Due to some issues with my flight I don't think I could start by the date indicated on the COS and am planning to let my employer know of this. So the question I would like to ask you is...

If I started a week later than the date on the COS, is this ok as it is within the entry clearance date?
Also, would it later become a problem when applying for ILR (thinking way ahead I know!) as I read somewhere that the delayed days could be considered as being 'out of country' or taking leave more than entitled per year? I need to read further into this but my energy has been exhausted in calling the UKVI everyday at the cost of £1.37 per minute!

If you take more than 28 days of leave per year, does your visa get curtailed? Someone has also told me this as well... Lots of questions, sorry but would love to hear your valued advice and comments. Thank you!