My MIL can be like this - it's a mixture of ignoring negative stuff generally, not being that interested in academic stuff as it's not her forte, and a belief common to where she lives that your either born academically bright or not and there is little you can do to change things.
It leads to thing like DC getting 4 out of ten and MIL going wild with praise while I stand there saying well done for trying - but you got more before so what didn't you get remember - what did you find hard shall we try again or look at that area. It's not that I'm not praising but I'm looking at why they struggled and trying to ensure they don't long term.
An entire report is to big an area - honestly best thing there is to praise.
You need to identify specific areas and work out how to help - and you can't do everything at once.
I always find summer is a crap time to get extra work done - the weather is nice everyone wants to be out and about. Little and often I find is key - find 15- 20 minutes a day most days to work on an area even during term time.
I think a selection of work books might be boring for both GP and DC - and if the DC kicks up a fuss about doing them are GP going to want t force the issue?
You can try making thing fun - make get IL on boards with a few tasks, if it's maths - lots of sites out there with games and stuff that help build core skills, if it's handwriting - pick out a nice example and see if you can get GP on board with doing 10 minutes a day with her - or writing lists/cards etc. with them. If it's reading picking out a fun book to read with GP - or getting them to do the library challenge with them.
I found I can get MIL on board with a specific task to do with the DC - especially if it's fun.
Lots of people do have idea that any extra work is bad - but as a child I really struggled in particular areas being dyslexic but was still academic. I don't want that for my DC. I've seen with my DC bit of extra help and support and they've gone from struggling to flying. That has certainly helped get my IL on board - even MIL.