Take all this with a grain of salt..... an overview, not the gospel for all cases.
First of all MANY expats at Lakeside do NOT use IMSS at all. Has something to do with income level IMO. Some number of people use IMSS for their 'catastrophic coverage'. Those that do use it often pay out of pocket until all of their benefits kick in. Many expats pay out of pocket regardless of their coverage.... or lack thereof. Many expats purchase private insurance, some of which also choose a high deductible and pay for lesser medical needs out of pocket. (Keep in mind that medical care in Mexico, even in expat-heavy Lakeside, is 'cheap' especially when compared to the US.) Private insurance has a threshold age.... 70, 72, 75?... after which one cannot enroll. Some/all? policies can be continued even after a holder passes that threshold.
Dental cost is extraordinarily 'cheap' compared to the US. A filling, a crown, a root canal, a bridge, dentures... even implants... can be paid for by anyone who has the means to actually be in Mexico to begin with! And Lakeside has several very high quality dentists from which to choose.