Do you realize marths grab hitbox lasts 2 frames and takes another 21 (30 total) to go back to normal and foxes dodge lasts 16 frames and recovers only 7 (22 total) frames later?
You have literally one frame of vulnerability before a dodge and 7 after. There is a huge window there to escape getting grabbed up close.
Im pretty sure Ive implied multiple times that this is all contengent of marth being close enough to shine.
Thanks for clearing stuff up.
My only issue with your reasoning is the main situation that this happens makes your choice much harder than this.
Techchasing on the whole is usually a combination of 2 actions not just one.
So say you tech in place and Marth is right there spaced and waiting. You don't have the time to decide whether he is going to grab immediately or bait the spotdodge/or whatever else he may have picked up from your pattern.
Same goes for most tech chasing . So you could say your already trapped by your frames before your option of spotdodging comes up. This makes it that jumping would not be viable if the Marth went to grab the tech in place.
Your only real options are shine(if close enough)/roll/spotdodge on immediate grabs
or anything else really if you believe they are trying to bait those options or they mess up the immediate grab timing.
Which is why it's really about prediction and reaction times. Not what you seem to be implying as a really safe method of escape.