You can not just cut a longer barrel this short, has to be a newly manufactured one at that length
Yes you can, if it starts out as non restricted and the finished product still meets the criteria for non restricted you can do whatever you want to it.
I think you may be thinking of the rules for restricted rifles. Can't shorten the barrel of a restricted rifle even though it should technically make no difference since it would still be restricted. Canadian firearms law makes a lot of sense eh? I know a guy that takes his 22 inch hunting rifles to a smith and turn them into 18 inch, this is no different and only a semi auto has to remain over 18.6.