You really need to keep an eye on the weight, so I agree with those who have said to buy food as you go. I walked from Sandwood bay up near Cape Wrath down to An Teallach last year (end April - best time!). I tried to cut weight to the bone, but had to carry 7 days food, as there are no shops up there. My food weighed in excess of 7 kg, and that was on the basis of highly calorific and weight saving dried foods. If you are carrying that much food and two litres of water, you are at over 9kg. Add in your rucksack at 2-3 kg and you have 11-12 kg without anything else!
I saved weight by using a space blanket as a groundsheet (80 grammes, but you have to be careful as flimsy), with no bivvy bag, but a tarp. I pitched the tarp using two carbon fibre walking poles from Alpkit, only 400 ish grammes for the pair, and very cheap (for carbon fibre). Ultralite pegs can also save a bit. A spare tarp is a luxury you don't need! I don't see why you would need an emergency bothy either if you are already carrying a tarp and bivvy bag. It's either/or in my opinion.
My advice is to look long and hard at everything, and see if you can make items perform more than one job, and whether something is really necessary. For example, biodegradable camping soap can wash dishes as well as being used for clothes, shampoo and skin!
Most people also carry too many changes of clothes. The caveat is that in April you could have glorious sunshine, or torrential rain or even snow, so you need to be sure you cover the bases. A decent set of merino wool base layers would help (and won't smell after a week, saving carrying too many spares), while a warm hat, gloves, and waterproofs are essential.
Enjoy! I'll be up there again this year at the same time as you, so I hope you get good weather...
PS last year the sun beat down for 7 days - unheard of - and we got burned. Take sunglasses and a lightweight cotton sun hat, and suncream. Seriously! My friends had to wear zip off trouser legs on their heads to keep the sun off as they didn't have hats.