How feasible to learn undergraduate math in one year?
I see that this question is being voted down for being interesting, so let me blacken my own reputation by addressing it.
Mathematics undergraduates do not study mathematics. They study studying, they study structuring their time, they study the work-life balance, they study manipulating their teachers, they study the opposite sex,… and in their spare time they also study mathematics.
As an adult you therefore have a considerable advantage over the children because you have studied all those other subjects and graduated in some of them. That means that unlike them, you can devote 100% of your time to mathematics.
On the other hand, there is a natural pace at which one can learn. Learning is not a conscious process but an unconscious one, and we have very little control over how fast the unconscious mind decides to process things.
Moreover, in mathematics 90% of one’s learning time is completely wasted - the remaining 10# then more than makes up for it. When you are learning from people and with people, this process adjusts itself naturally. When learning in isolation, it is not clear that it will. You do need someone to guide you. The guidance is not actual tuition (as it would be if you were an 18-year-old child) - more, it is mentoring. Your guide will tell you things like “yes, that concept is difficult” or “this book’s approach does not suit you: try that one instead”.
So think hard about who might be able to guide you. It doesn’t necessarily have to be someone senior. I don’t know enough about your situation and environment to be able to advise.
BUT I have read your question a few times and I cannot understand why you are specifically talking about one year. There may be good reasons for it - but without those reasons, it sounds unnecessarily restrictive. You will certainly be able to learn some one-semester subjects in a couple of weeks. But there will be others that will take far longer than you expect.
Overall though: the signs are good and don’t worry about being too old!