I empathise - we don't have the "luxury" of DMIL coming to visit us, we have to go to her still. (DFIL died 18 months ago, DH and DSIL are only just getting to reduce alternating weekends visiting her, while DBIL, well, I'm not quite sure what he does other than expecting her to continue doing the school runs for him and not get her hearing aid appointments because they clash with school run times....he lives next door!).
DD and I have not gone all that often - we did a lot while DFIL was ill (5 weeks) and in the immediate aftermath. And every single bank holiday for over a year, last Easter, both Christmases, etc. etc. But DD has Cub Scouts on Saturdays, and I am a leader there, so I can't take too many weeks out, meaning we have stayed here without DH many other weekends (and as he travels at least 1-2 night every week for work as well, that's a lot of abandonment that DMIL doesn't recognize). She also doesn't want me to take time to see my family when we visit (we grew up 15 miles apart, which is 250 miles from where we live) - not that she doesn't want me to see them, but manipulates it and has reasons why I can't go, or makes me feel guilty for going out for 1-2 hours of a weekend visit (Fri evening to Sun afternoon, staying in DMILs).
So while we went down for Christmas, we went back to what we'd done many times in the past - rented a cottage (Christmas-time its necessary, we've found over the years - the rest is fine to stay with either family). This Easter, we will visit but spend more time at my family's holiday house, which has always been the Easter tradition. And we didn't go down for the last bank holiday earlier this month - which was probably the first one in 18 months that we spent in our own home.
In your case, I would suggest that you do ask her to change her dates, and only come on Saturday. Do a day out on the Thurs/Fri with your own family - let your DH get out into the Fells and have some time to get to grips with HIS loss as well, and you all get the fresh air and exercise that it sounds like you really need.
Plan ahead a little for Saturday's dinner, but ask for DMIL's company in the kitchen to prepare it. Or even get DH to prepare it with her. Look at the tv schedules in advance, and if there's something there that YOU or the FAMILY would like to watch, say it early on (not immediately on arrival, but when you are sitting over a cuppa maybe) that "oh, by the way you hope she doesn't mind but we saw that X is on at 8pm, and we are really looking forward to it so we'll be changing the tv channel at that stage, would you like a slice of cake that DD made/went and bought especially with your tea..." moving swiftly along to other matters, fait accompli style.
If a DC has to sleep on the couch, then I'd be inclined to move everyone else along to bed no later than about 11, because it has been a long few weeks and everyone needs a good sleep. (Perhaps mention this earlier as well). Make noises like "DS you are looking really tired, would anyone, including you DMIL, like a hot water bottle or Horlicks or hot chocolate etc before going to bed?" at maybe 10.45 or so, to start everyone moving along nicely. (Could the DC sleeping on the couch be persuaded to start yawning on cue?!).
Sunday - yes I know it's a pain, but a "proper" Sunday lunch should be organized - whether that's at home or somewhere nice but reasonably local. If you are likely to get comments on Saturday evening, you could do a lot of the prep in advance (perhaps even with all the rest of the family involved) for peeling veg, making stuffing if you are so inclined, doing dessert etc. You could set the table for breakfast. Those sorts of jobs - and get the DCs involved as well as DH.
Sunday morning - I'd suggest either going to Church (if that's your thing, particularly in light of DFIL's recent death it may be something some want to do), or getting a nice bunch of Sunday papers and some nice croissants etc for a good breakfast, sitting over coffee and relaxing. If cooking roast dinner at home, do a version that requires meat in and then ignored for a long time, then a tray of potatoes and some veg doing late morning - which can be prepped in advance or done last minute. But take time to slow down in between.
Maybe go to a garden centre in the afternoon, as much to see if there's anything you'd like too, now that spring is coming. It might be nice to see if there's a favourite plant of DFIL that either your family or DMIL or both may like to have this year.
Again, have something noticed on tv for a part of the evening, or maybe suggest a board game/cards or something together instead? And have another early-ish night.
While not rushing DMIL out on Monday (or Tuesday - depending on what suits you best), it might be an idea to have something planned for the afternoon so that she is organized to go straight after lunch. I don't know if that's DH driving her back, or someone dropping her to a train/bus - but that you and the DCs have a planned activity (whatever that is - it might be needing new gear and planned time to get to the shops - I know that's always a hard one to fit in here! Or an actual outing like the cinema or an outdoor activity or a planned meetup with friends etc). You would know best if DH should be part of that - I am only not including him as I assume he'd be on logistical duties. Maybe keep the GC outing for Monday if she's staying until Tuesday.
I hope it all goes well. I hope you all get the break you need, and that DMIL can come and enjoy some time too.