American with Mobile US office staying in Canada?

Here are the details:

My boyfriend (US) and I (CDN) have been dating for a year and 5 months now. I am in Vancouver he is in Bellingham and we have been going back and forth visiting mostly for a 2-3 days at a time (ie. over a weekend, etc.). We both have Nexus and neither of us have had a issue at the border and we would like to keep it that way. However we are growing tired of the back and forth and have been considering him coming to stay with me for the max (if allowed) 6 months.

I have a great desk job that would not allow me to visit down there for extended periods of time however he works from home for a magazine and is able to bring his office anywhere. I know that he is not able to work in Canada for a Canadian company collecting Canadian dollars but is it legit for him to work from home but it's just my home? He would keep his American bank accounts to which is paycheque would be deposited, he would just be visitng and staying with me.

As a second question, he wouldn't be able to produce a lease or mortgage document tying him to the states (a reason to return) and doesn't have money in the bank or a lot of room on his credit cards. I am in a position to float his stay but what are some other things we could provide as evidence that he will return and should we only produce this if asked or should we have all this ready the day he wants to cross and preemptively hand it over?

As a third question, I have read on this forum (so very helpful BTW) that visits can be extended, PR can be applied for, etc. to extend over the 6 months but what I am unsure about is the amount of time he would need to return to the US before he could come back to visit again. If he stays for 6 months does he have to return to the US for 6 months before he can come for another visit - no matter the length?

Thanks so much in advance for all your help!
kb