You're exactly right. At this point, there's no point in wishy washing about whether to file a complaint or not. Because if you're fired, you're fired. If I were you, I'd file grievances with EEOC and ADA myself, regardless of what your union does for you, because you have absolutely nothing to lose. Don't worry about retaliation, what're they gonna do to you, take back your pay retroactively? Fire you twice?
As for worrying if you have a valid complaint, don't be hesitant to file with this much of a case, because the folks at EEOC will get paid, whether they determine your claim is valid or not. The downside is that it may take a really long time, six or eight months at least, for them to get to you or make a decision, but file quickly, because there is a statute of limitations.
If you can afford it, you may want to talk to a labor attorney about your termination. Or legal services, if you feel you qualify for this sort of help. How about that union you've been paying dues to all these years? Some of them will assist members with such cases. Who cares if they aren't willing to go all the way with it or something? Do it anyhow.
If the attorneys you talk with politely tell you "someone" may be interested in taking the case, but not they're too busy, don't do this type work, etc, you probably need to accept that you don't have much of a case for age or disability discrimination. We none of us here can tell you definitively.
First, though, and totally unrelated, as soon as you are fired, file a claim for unemployment benefits. You do this the first week you are not working, doesn't matter if you have enough money to live on right now or you're a proud soul who never wanted assistance...do it anyway, it's not welfare, it's yours there to draw by federal law if you qualify. You will set up a claim based on your past 18 mo. to 2 years of work, and if you tarry, those wages will be gone, and you will not be able to draw unemployment ever, even though you've worked all these years.
Since you were fired for misconduct, you will explain the exact circumstances of your termination, explain what the rules were, explain whether you had had any warnings or write ups for THIS particular thing you did. Be sure to mention that you'd seen many other people do this with no repercussions. Be sure you mention that you'd been out of work, that you have seen a trail of discriminatory actions toward you since you have begun to get older and have health issues that forced you to be absent a lot or that they were going to have to make some allowances for to continue your employment.
Since unemployment insurance IS NOT just about discrimination, and has nothing at ALL to do with either your union grievance or your EEOC filings, you do not need to go any further with those issues in filing for benefits. Your employer will then be contacted and asked for the reason they say you were terminated, but what you say will be just as valid as what they say. They do not make the decision about whether you get to draw unemployment or not. As I said, you may be denied the first time, but later approved when you have a better case and they cannot produce proof your firing was for a good misconduct cause.
Okay, be sure when you file you tell the unemployment claims office you are able, available and actively seeking other work. DO NOT tell them at the unemployment office that because of your diabetes, your sleep apnea and mild hypertension you cannot work or cannot do some types of work. This is an insult to people working everywhere everyday with these types of conditions. Do not try to put health issue restrictions on yourself at this point. They will not offer you any work that is like ditchdigging if you were not a ditch digger on your former job. Even if they did offer you a job you did not feel you could do, you could go on the interview and consider it before you made that decision. It sounds as though you were in the industrial field, you must be able and available for work in a similar field. The place to discuss these sort of issues is with employers, NOT with the unemployment office when you are signing up to draw. Just state that you are able and available, and willing to participate in any job search activities they offer you.
You will very likely be denied initially on your unemployment claim. This means you will have a hearing, and will be asked to present your case to an appeals referee. They will be determining if you were terminated for a valid job related misconduct reason. They will determine if the reason you were terminated was "gross misconduct" (so bad you should have known it was wrong, even once, to do it) or else that they had a clear pattern of verbal warnings to you, then write ups to you, and then you still chose to do the thing that got you fired.
Of course, you will have some very good argument to make that this was not the case, that you were fired because they wanted to get rid of an older, slightly disabled worker. That you were still doing the job to the best of your abilities (This isn't the time to mention that due to your health problems, you were slowing down and could barely do anything on the job(!)) and that you did not want to lose your job.
But now you are terminated. Your employer is not big in your life any more. Your union, your co-workers, all this is over. No appeal to anyone for any reason is likely to get you your job back. It's time to move on. If your health problems are such that you do not expect you will be able to get another job, you may want to think about beginning the filing of a claim for social security disability, though you have to have been out of work for a very long time before that would be a possibility. And until it is approved, you can be doing work searches and drawing unemployment insurance if you get approved.
Get busy. File for unemployment, get that started. Talk to attorneys, even without one, file your claims timely for EEOC and ADA issues. Then begin moving on with the rest of your life.
By the way, it is my opinion that if your employer spends a lot of time browsing these sites, and might find out you're posting and ...I don't know, put out a contract on you or something, then he isn't much of an employer and doesn't have much to do!