Hello @mzvib,
Q1 - Is it safe to add the new line before the porting out is complete? If I have to pay double for a day or two, that's fine.
Sure you can add a new line before you port out the existing number, you just have to make sure that the existing number is active before the port out happens.
Q2 - Will the porting out be treated as a cancellation and subject to a 30-day notice requirement? It is a BYOP plan with no contract. Since my intention is to immediately replace it with another Fido line, my hope and expectation is that there will be no cancellation fees or notice requirement. (Not one Fido rep among the 4 or 5 I've chatted with this week mentioned anything about notice, but I just want to make sure.)
The port will automatically cancel your line, because its a BYOP you do not need to give notice and there is no fee.
Q3 - Does it make any difference if I add the new line after the porting out of the old line is complete, probably later the same day? (Other than a few hours of no service on her phone, of course.)
There is no difference if you add the new line after.
Q4 - To minimize downtime, what's the quickest way to activate the new QC line and get a new SIM? (Since her current SIM will stop working as soon as the number is ported out.) We will be travelling this weekend so delivery isn't really an option ... would prefer to just go into a Fido store and have it all done at once.
I would suggest you buy a new SIM card and call customer service and they can activate the new line with a Quebec number for you.
Q5 - The online reps who offered to activate the new line right away (I balked because I thought that would screw up the porting out) weren't concerned about proof of QC residence for my daughter. But if I go into a Fido store in Toronto, will that be a problem? The only thing she has at this point is the email from the university residence confirming her room number and move-in date. And will it be a problem that the billing address for the account is in Ontario? (Again, the online reps said no problem as long as she's getting a Montreal number, but I don't want any unpleasant surprises when I go into a Fido store -- I've had enough of those in the past where they told me something completely different from what a rep had told me on the phone or in a chat.)
It would be best to do it through customer service since they are better equipped to handle these type of situations.
Q6 - If we activate a BYOP Pulse plan now, can we get a subsidized phone later by adding $15 or $20 a month to convert it to a "medium" or "large" plan, and otherwise keeping the same plan at the same price? Or do I have to add the phone right from the beginning?
You can activate a BYOP plan for now and at a later date you can change it to a small, medium or large providing the plan is still available because offers do change from time to time.
Hi @mzvib
Thanks a lot for reaching out 
I'll be able to answer all your questions here!
Q1-Q2-Q3
Porting a number can take up to 2.5 hours before it's completed. In most cases, the port is done within a few minutes. You can add the new line, before or after the port, it doesn't change anything.
Since you have a BYOP plan, no notice is needed, you'll be billed until the day that line gets ported out.
Q4
You have 2 options, if you have a SIM card simply contact us here and we can activate the line for you. Otherwise you can visit a store to complete the activation.
Regarding the port-out request, we have no control at all on the number porting out. The request will be triggered by the other provider (line must remain activate on our side when you initiate the port-out request).
Q5
No worries, as long as you get a 514/438 number she'll be eligible to a plan from Quebec, no proof of address is needed for that.
Q6
There is a minimum wait period of 30 days from the original activation date to order a device if you first activated a BYOP plan.
You'll have to update your plan to a Small, Medium or Large one in order to get a discount towards a new phone.
Let me know if you have any other question for us
