BigBub

Since it looks like your property was built from a division of land from the property outlined in blue, I would check first that PD rights weren't removed.

It would be worth checking if your LPA offer a duty planning service. Some will offer advice over the phone or with an in-person meeting and can give informal advice on your proposed development/drawings. There is no charge for the duty service, but not all councils provide it.

Definitely try an appeal, there isn't much to lose. If you decide not to appeal then you could also look in to making a complaint with the local government ombudsman, although they can't change the outcome of the application, they can force the council to pay a financial remedy if they deem you were given any of the following: inaccurate information about procedures or planning law misleading advice in advance of making an application, or no or an inadequate response to correspondence about your development proposals before an application is made or determined. https://www.lgo.org.uk/make-a-complaint/fact-sheets/planning-and-building-control/how-your-application-for-planning-permission-is-dealt-with

On your first and second points, you should have a look at your local planning authority's Validation Checklist which sets out the local requirements for drawings and supporting documents needed for an application to be validated. If the drawings did not meet all of the validation requirements then you can raise this with the case officer to have the applicant revise the drawings.