Oh dear, Arwel - that's an awful lot of Marmite. Will the smell (and taste) dissipate? I honestly haven't the foggiest. However, the blackberries may mask it a bit

. I'd just let things run their course and then see what you think - you never know, you might like the flavour of Marmite wine.
The differing amounts of juice will give you different wine styles. Mine is a light aperitif, FTBs is a full-blown, in your face version. You can do that with any juice (except maybe apple) - a half-litre for light, up to two litres for heavy flavour.
Your reasoning for the blackberries, by the way, was way out

but the effect may be good. It's part of the fun when you're designing recipes - working out how much of one flavour will go well with another, or even if they'll go together at all. As a general guide, adding sultanas or raisins (raisins for reds) is always a good idea as it adds to the wine character - it's much the same (but not quite as good) as adding grape concentrate. However, at least half of the weight of any added sultanas/raisins is sugar, so you have to take that into account.
Anyway, I'm keeping my fingers crossed for your grapefruit/Marmite, but I can't say I'm optimistic.
Mike