Your tips on how to keep to bcamp when staying with relatives and friends will be much needed
Do they know you eat low carb, high fat? They should be able to accommodate that fairly easily--much of this way of eating can be achieved by simply not eating parts of a 'regular' meal, like skipping the potatoes at Sunday roast or having just the egg and bacon but not the toast and baked beans at breakfast.
It depends entirely on whether they're planning to serve meals that you can also eat or adapt.
Most people are supportive of you ask them to be. If they're not, then you must plan and provide your own food to stay on track.
And cake is easy, just politely decline.