Hi Laura and welcome to the forum, and sorry that you have found your way here in such horrible circumstances. My condolences for your losses and we realise that you and your mum will be lost in all of this and just need a bit of advice and pointers. If you could help with a little bit of information about the boat people may well be able to help by pointing out dry-docks, cranes, capable yards / workers etc. nearby.
The key question would be, do you know what needs doing to the boat? Was there a survey done, where your grandad was going to address the problems it raised or are the problems plainly self-evident. Are the major problems engine, hull, electrics, windows or a bit of everything? Were any words like 'overplating' or 'engine rebuild' mentioned? They are expensive words.
What type of work was the 'cowboy builder' supposed to be doing before he did a runner? When you say a lot of expensive stuff has been taken, what type of things are they. If it's some nice brass lamps they can be lived without, but if it's the gearbox, the boat's going nowhere without it.
I'm slightly confused when the thread title says 'sinking' but at the end of post #1 you say you want to get it back afloat. I was assuming that you were meaning that it was in the canal already, in a state where it needs attention, and the 'back afloat' meaning in decent usable condition. Is it correct that it is in the water at present? Talk has been made of getting it out onto the hardstanding to work on it. This may be necessary for some of the work but the boat is going to have to be able to move at least to a place where it cold be craned out.
Are you practical minded yourself with the time to do any of the work. If you can do some of the basics like sanding and painting, it won't be the aesthetic standards of a professional job but it will save a lot of money and be acceptable.
Whatever the boat needs, even if it doesn't need to come out of the water for work to the steel, it will probably be well overdue for drydocking and blacking. Doing the blacking yourself can save several hundred quid. However, spots for dry-docks are usually booked well in advance and this lockdown will have caused much disruption to their calendars.
Please don't despair about the situation. If we work out what questions need asking, we can work out what issues need fixing and where you might be able to find skilled and reliable assistance without breaking the bank.