If you made timber stud walls and had the additional piece of roof mostly or all clear then you would still benefit from shade of the Wisteria in Summer. There are various clear roofing systems at different costs. Some access points (like rood lite windows) would be a good idea in two or three places, to prune the Wisteria. I built a structure in my garden using mainly reclaimed materials from a house demolition. Only the screws and clear roof panels had to bought. The wood-burning stove and the correct chimney with attachments is likely to be the most costly bit.

Both forms of Wisteria, floribunda and sinensis have a vast range of colours available and both will produce racemes of up to 30cm in length. However floribunda can produce racemes of up to 100cm in length and does need a very large wall to show off its blooms. Wisteria sinensis produces its racemes before the leaves develop so shows off its display better and is also better suited to a house wall. Buy a named variety and not one just grown from seed as these can take over 5 years before they produce flowers. Varieties like 'Prolific' and 'Alba' would cover your two colour choices, but the sinensis 'Alba' has quite short racemes, if you want really long white racemes then I would go with the floribunda 'Alba' but this is a very vigorous climber and if you want to keep it in check you will need to prune it more than the twice a year recommended but the books. This is the Wisteria floribunda 'Alba' in my own garden, as you can see the flowers are out at the same time as the leaves. It has taken me 15 years to train a small plant in a 5ltr pot into this standard. And this is the same variety I trained around my pond. This one is a sinensis over a pergola near my Betula utilis jaquemontii also in my rear garden. As you can see I like my many wisterias in my garden. Hope this helps.

You have to basically cut out a hole in the wall that is as wide as two doors, then install the pocket door framework which can come in a kit form - one half then gets covered with plasterboard so that it is inside the wall, as the pocket where the door slides into when you open the door. The bit around the actual door then needs finished to whatever specification you want - usually with framing etc that a 'normal' door has. Depends on the construction of your current wall though - whether it's load-bearing and therefore you'll need some sort of beam to support the wall above. Or if it would be simpler to box out the rest of the wall rather than cut out a massive hole - in this case the room would get a bit smaller. If you are cutting a hole in a modern stud wall then it won't affect the size of the room. Door handles need to be considered carefully as you need something flat that can fit into the 'pocket' and you also need a fitting on the visible edge of the door to pull it out of the pocket again to close it.

If you cannot reach agreement to do this work safely with the structural engineer that you have consulted then I suggest you get another one to give you a second professional opinion. You should never attempt this type of work without a professional design and calculations. You do not know the design details of the finished example you are viewing although I agree it does look possible.