I'm another one who had a chocolate wedding cake and served it as dessert. The cake was amazing - my mum had had her cake as a dessert at another wedding and was raving about it and it was truly amazing (she used cream in it - there was over a pint of cream in the cake mix - a secret family recipe and I've never been able to track down such a good cake recipe since!). We served it with big bowls of raspberries, scoops of ice cream if wanted and jugs of cream and it went down really well with everybody - there were no complaints from young or old. Including old codgers that you might have thought would have complained as they wanted a dessert and a slice of fruit cake later.
We had plenty left over - froze the top layer and used it at my dneice's christening a couple of months later. Used some later to have as dessert second time around with the evening 'meal' and took the rest home and gorged on it - plus sent chunks home with assorted family members for them to enjoy.
I'd always known that I didn't want fruit cake as a wedding cake as I loathe it - the other thing I did was go to one of the big wedding shows and try lots of samples. The one I almost went with was with a big tower of individual desserts made by Eric Lanlard - he wasn't famous back then well I'd never heard of him! the desserts were delicious and the price was really reasonable - same sort of price the caterer would have charged to do a dessert this is going back 11 years though. People are used to seeing giant cupcake towers these days so think of it a bit like that but with lots of beautiful individual desserts instead, that could have flowers and ribbon threaded through the stands but didn't need much as they looked wondrous anyway.
With either option - we figured that we saved loads as other than a token 'corkage' fee (plating up fee?) we didn't have to spend lots on both a dessert and a fruit cake.
With regard to our supper buffet, the caterers were really keen to sell us a fancy buffet. We just kept it really simple - loads of mini sausages, rolls, veggie pizza and a cheese board, with apples, grapes and more wedding cake for afters. People didn't want to eat loads but enjoyed having something tasty and simple to nibble at. And again it meant it was cheaper, we were able to say we wanted to take home the big blocks of cheese that hadn't been eaten/give to family to eat up, ditto everything else, rather than watch them throw out (or eat themselves! - we overheard one of the waiters grumble that they weren't getting doggy bags of nice food to take home as usual...). Helped that we weren't off on honeymoon for a couple of weeks so didn't have to worry about that side of things and were there to eat up the bits.
So no - guests will not feel they are being short changed - they will think you are being eminently sensible and they'll really enjoy their dessert. You could present your mother with one of those teeny one-person sized wedding cakes so she could have fruit cake if she really really wanted some!