Post base (deck) question, job tomorrow, looking for input today

Thanks all in advance!!!
Hello - I am installing a deck railing system for someone who is very particular. Their existing deck does not have a proper railing system. This customer is very particular about where posts should go, how they line up with other posts, etc. All should be good except one post at the bottom of the stairs. There is no way to attach it to the stringer and have it line up with the railing at the top of the stairs. I do not think that scabbing on about 4 inches of material to the stringer would be suitable or stable enough.
My only solution, I think, is to set the post, at the bottom of the stairs, on top of a concrete footer. OK - frost line is published as 48". Do I really need to worry about frost line for such a situation? I would like to just put in the footer and then attach a post anchor to the top of it once the concrete cures enough. (Helps with alignment). The footing needs to be square, don't ask, so sonotubes are out, which is fine with me. I was thinking I would dig about an 8" diameter hole and set a form for the required shape at the top. Question is, how deep? The area is very rocky and wooded. I expect to bottom out on a boulder or major tree root pretty quickly.
Questions:
Would 18 - 24" deep by 8-12" diameter concrete base be sufficient for a base for a short post to hold one end of a handrail when frost line is said to be 48"??
How are posts at the bottom of stairs usually handled? Are they attached to stringers or set on/in concrete?
If I were to use blocking to allow the post to attach to the stringer do you think this would be sufficient? Bolt through most likely, I would assume.
Some more details. Material is Mahogany Balusters are Dekarator aluminum posts are 4x4 half lapped over rim joist attached with carriage bolts, outside corners 1/4 lapped attached with lag screws.
Thanks again!
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What is the footing of the base of the steps? If it is just a floating slab then I wouldn't worry about going down below the frost line, just make a 18" deep block to attach the base of the post to and be done. If the pad at the base of the steps is floating, you need the post holding the base of the railing to float as well or it will rip loose this winter when the base of the steps frost heaves upward and the railiing doesn't.....
You will still probably need to attach some blocking to the stringer to the post just to keep the steps fixed to the post properly if you know what I mean.
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Thanks again - For future benefit to others here is what I did. Dug down next to stair about 18" deep. Discovered a concrete base under the stringers. This base was only about 14" deep and was not build with any form. I made a U shaped form and pourde my concrete right up against the existing concrete for the stair stringers. The thought here is that if one moves, so does the other. OK, Now Once the concrete set up I drilled for an anchor and used a bolt down post plate. I was also able to put in some blocking and use two carriage bolts into the stair stringer. It doesn't wobble but I do not think it would take 200lbs of lateral force at the handrail either.
What would have been a better way of doing this? I suppose rebilding the stairs so the stringer lined up with the post plus using one of those steel tube post bases you bury into your concrete.
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