Hi Kate,
I have a family member who claims enhanced for both who suffers with Fibromyalgia, so I would persue with this.
1/ I would say make a bulletproof list of points that you think the assessor could ask. Unfortunately, there isn't no guarantee on the questions, so it's just guess work, but all you can do is prepare best you can. I would also say to use the guides on this website, to help advise you and prep you ready for this.
2/ Yes this can be done. My family member who claims for Fibro ( as mentioned above ) - he tried the assessment and found it way too overwhelming, so his partner had to deal with it on his behalf.
Hope this helps - Chris.
Nothing on this board constitutes legal advice - always consult a professional about specific problems