Nice to see you are up for a challenge working with a mature client with very common mobility limitations. (Good on her too for wanting to stay active)
It's not that she's not interested, it's that she physically can't stomach them. The large fish oil pills make her gag and the fish oil liquid is even worse (even the Carlson's lemon-flavored). Sigh.
Given her knee pains, migraines, etc. I think that it's absolutely worth it to her from a quality of life POV to take more fish oil. It's just that she doesn't like pills or the liquid. At this point, the athletic endeavors are far less important and are not a factor in terms of the fish oil.
I respectfully disagree with the benefits of fish oil for everyone, but I'll stay out of that debate since I know it is currently very popular and well supported by users. If people believe something is working for them then go for it.
Swallowing large pills, capsules and softgels are an issue and can be scary if you've ever had one stuck on your windpipe or scratch your throat going down sideways usualy the result of getting anxious about taking them. I've been known to chew them and wash down the nasty grit rather than stressing over them. Probably it's an age-developed thing. When I reflect back on all the dessicated liver pills I took without a flinch back in the 80's when they were on everyone's basic supplement list, I had no problems. Today no way.
Another technique I've learned is to hide oils/liquids in my food when my palate just didn't want to swallow it. (I used to mega-dose (3-6 tbsp per day) on flax seed oil when I had a nasty skin condition that Rx meds couldn't resolve.)
Maybe your client can learn to do the same with fish oil by putting it in salad dressings and sauces. I ate plenty of salads and cooked veggies topped with a blend of olive oil, flaxseed oil and lemon juice. Also butter and oil mixtures or even home made mayo with flaxseed oil as part of it. There is nothing like upping her salmon intake either. That would address both fish oil and protein issues.
With regards to the slow progression in making gains...
One trainer I know had a middle-aged male client who was looking for PT just for general fitness to support weight loss. This man did not even have the strength to hang from a bar to do an assisted pull up when they started. That became the goal ...to hang from to bar first. Gradually they worked towards one pull up as a goal but they didn't do them specifically the outset.
I think you should plan your weight increase progressions based on how her last performance of the exercise went, not by expecting her to be at a certain weight by the next consecutive day. Lots of trial and error. If she's having a good day, up the weight. If its a struggle then dial it down for the day.