OP, it sounds like a distressing situation, I can understand why you might be at your wits' end.
I'm hopeful that you might find an agent fairly easily though. Simply google the location and check google maps reviews for the first 5-10 results, pick a few to call and see who you get a good feeling from.
Almost all EAs I have dealt with have had strong individuals (often the older ones, as PP has said) on staff who were good to deal with, knowledgeable and reasonably expert in the local properties and their potential.
It's just a residential house with land, neglected, with an elderly eccentric owner, and you want their advice on what can be achieved without any renovations and in a fairly quick timeframe.
You could take photos of the current situation and share those as the basis of discussions before choosing on an agent to meet the relative in person and discuss how to get the house on the market.
They'll most likely have sold 10-20 comparable properties in the past year - I understand you are dealing with an extreme, but in essence it's not an unusual situation. The EA will have a clear professional incentive to give you and your relative all the support they can.
Side note: if things progress, you may want to discuss with your relative if they are happy to authorise you or another trusted person to be their representative for the sale. We did this for my MIL when she down-sized. She did put in a fair few calls to the EA and solicitor but because we were the 'point of contact' she was largely protected from the drama and angst...