Extra direct trains from Edinburgh Waverley to Carnoustie do operate during the Open but, as noted above, they are pretty busy. There are always the indirect services...it's a good service and not a big hassle.

HIEX City Centre is a good property in an excellent location (and it's much less than 10 mins from Waverley...it's just that walking in Edinburgh often feels like a hike because you're slogging uphill!) Even with the higher redemption, it is a good use of points - room rates in Edinburgh are steep.

A good alternative would be the HIEX Waterfront down in Leith. Less attractive and further out, but it is well-connected by good public transport and road access and it's cheaper.