The spam attack is worrying for me because BIP101 can do nothing about it as the Jan date is hard coded.
It's hard coded that way in XT, not BIP101 itself. BIP101 could be implemented in Core overnight. Or am I mistaken?
The Jan date can just be changed, it is just code.
The BIP101 can be added to the Core with just a different date.
does anyone of you know some details of how it is implemented?
eg. (to take your example) BIP101 would get implemented in core right now but no pool would produce a bigger block until january.
in january one pool mines a bigger block: would core and xt be compatible in this case?
(the answer of this interests me also for all different kind of blocksize-bips)