Make the idea of spot welding the last recourse, you might get some weld spatter stuck on some parts of the arbor
shaft thread, that will be hard to remove unless it is well rapped or covered with masking tape.

May I suggest to try first using a vise grip on the arbor nut, adjust it to a good tight grip so you can turn and loosen
the nut from the arbor shaft, some slight hammering or tapping of the vise grip might help in doing the job.

For the allen socket, will a bigger size allen wrench not fit on the socket ? Instead of welding something at the end
of the arbor shaft, try using a metal epoxy or JB Weld, the one being sold by JayL, fill the socket with the epoxy
then insert the allen wrench then let dry for a few days before trying to loosen the arbor nut.

​Hope this can help.

thank you bro rosy on your tips and suggestions especially the weld spatters.
i dont think the jb weld can hold for that purpose bro...
the allen wrench 8mm in size still fits the socket but the recess for it is shallow enough to lose its hold while turning...the depth of the allen socket is maybe around 6mm only...this is a design flaw..i would have preferred a double wrench system instead.
my last recourse would be to remove the cast iron top so to expose more of the arbor and to explore more possibilities..but i cannot do this yet.

just an update....i was able to finally release the arbor nut by sacrificing the old blade and use the blade as a lever to unthread the nut with a few hammer whacks.

Well that is really great! Good for you, now you can replace the blade with a much better one and with more teeth. 😀

Best way is to use an impact wrench. Check the best cordless impact wrench for the money.

As others said the best choice is an impact wrench. If it is hard to remove choose one with the most power. You can read about them and choose the best one on https://www.insidetool.com/ .