Thank you so much for all this valuable information.
Based on what you told me and the fact that we are going to Vegas with my in-laws who have limited walking resistance, I think it would be best for us to stay somewhere in the middle of the strip.
Treasures Island could be a great option (what do you think?) as it is more or less in the middle of everything and it even offers free parking for guests. Even though it costs a little more we could take it into consideration for its location. I've seen good rates on Hotwire for the TI (the amenities and the TripAdvisor rating/number of reviews suggest the offer is for this property).
Harrah's could be an option too.
I don't see any offer for it, though as the only 3.5* that shows up at $69/nt, looks like the Stratosphere (amenities: Free parking, Free internet, Resort, Casino, Smoke-free rooms, Fitness center, Pool(s), Restaurant(s), Internet access, Golf nearby, Accessible for visually impaired, Accessible for hearing impaired, Accessible path of travel, In-room accessibility, Accessible bathroom, Accessible parking, Roll-in shower) which, after your considerations, we are not considering a viable option anymore.
Although I have no experience with these properties (I have stayed at the Mirage, the Luxor and the MGM Grand before) they seem like acceptable options considering we have no big necessities: all we need is a clean nice room in a comfortable position so that my in-laws can enjoy their stay without getting too tired. We don't gamble (of course we will, just for fun, but it's not the goal of our trip, as Vegas is just a stop on our way from LA to Yellowstone NP) therefore we don't care about how the gambling halls look like or anything.