The Army rates on unfit conditions. As a general rule this matches up to what your P3 profile is for. While there are routes to fix it if your profile isn't for the right condition(s), you're at a stage where it makes sense to ensure your profile is correct to make everything easier.
If your back is the only thing profiled, most likely you will receive 20%. Usually they have to measure your forward Range of Motion at 30 degrees or less for a 40%. 30 degrees is not much movement at all, that little movement suggests you're experiencing significant difficulties getting your boots on and off. While a 40% isn't unheard of by any means, especially after years of service and avoiding MEBs, allowing the problem to develop beyond a normal unfit state, it is uncommon.
The stuff with your leg suggests you have nerve involvement in your back. This creates a separate rating, but often the docs do little workup on it, primarily because there is little treatment options for it. That separate rating won't factor into the Army rating unless you can show it separately interferes with your work (think profile again). The MH, knee, elbow and shoulder problems have likely existed for awhile. While they have probably interfered with things, if you do the typical profile avoidance Soldiers do, that will suggest that they have not. Again, think profile to document those limitations they've created.
Its hard to say much about the actual chances of retirement without knowing all the details, which likely won't come out until the MEB is started and you have your C&P exams. I just know most Soldiers with 15 years in have avoided getting medical addressed because they don't want an MEB and then when it finally gets too bad, that lack of documentation works against them. Its a fixable problem, you just have to get whiney for awhile.
Its worth noting that making it to retirement is better, even if the MEB gives you retirement. CRDP allows significantly more money if you make it to 20. I think the doc bringing up the MEB is a hint that things aren't going to get better. I agree, the idea of a MEB isn't fun, and if you can avoid it, its to your benefit. Just be careful that avoiding it doesn't cause you issues due to being too good at hiding your symptoms, but not quite good enough.