You could go wider but it depends on what type of dining furniture would want to use and how many you will be catering for. Bench seating tucks underneath and when it is being used you aren't so worried about the other space. You could always turn it around the other way [ perpendicular to the house] so the width is less of an issue as 10 metres is a decent length deck. I think even an extra 30cm would make a difference and not doing it may cost more in regret than the materials. You could go wider in the dining area only but it is a bit hard to judge without seeing the space. Find some furniture you like online and map out a plan for yourself - use newspaper for life size modelling. Good luck

I've coated many decks and the best advise I've been given is "leave it natural". Most timbers go grey after a few months and don't look too bad. If you want a bi-annual maintenance job the other bit of good advise I've had directly from the manufacturer is put down four coats minimum. The third and fourth coats won't use so much oil because they don't soak in. A very light sand between coats keeping in the direction of the grain is a good idea to retain a finish without wood grain or dust spoiling the final appearance. Two coats, no matter which brand or type of oil will last about a year in our sun.

without the plan of upstairs i'm not sure that my suggested changes to front wall will be suitable but instead of reducing the rear deck, taking space off the front porch would allow extra space for the kitchen without changing the front set back and, unless there's some special reason for the extra toilet could you consider different access to the downstairs bathroom to have a more private toilet with included handbasin with or without the extra sink in the dining area?.. there's enough space for extra storage for utility and dining area with a table for two in the kitchen and extending the floor of the deck to the rear and side fences would allow more outside sitting area

What sort of easement? You can plant on it but research first so you don't plant anything with aggressive roots. Our water utility used to have a list categorising plants which are safe, plants which should not be planted within x metres, etc. Check if there's anything like that available from your water utility or council. If the plan does not have indicative furniture placement for anything else I don't know why it would show a dining table. Solar HWS or solar panels? Plants as large as trees down the narrow space between your house and the boundary with your neighbours create potential problems with roots and with leaves in roof guttering for both of you. Double glazing is valuable, especially in your winters, but for summer you want to stop the sun from shining on the windows in the first place. It might not be the most chic solution but when we needed something temporary and economical we used the shadecloth screens from the hardware store.