I’m sure somebody else here can give you a better definitive answer than me on the legal position and some general ideas of experiences, but meantime ...
The retailers you mention are very reliable along with others like Net-a-Porter and Matches Fashion. I’ve sent high-end bags with defects back to both Farfetch and MyTheresa, but to date only unused within the returns period. I received prompt refunds/replacements with no query. I never worry about buying from these good retailers and I believe from my experience of how they operate these ones (all of whom I buy from regularly) would stand by you as a matter of courtesy in the event of a problem appearing later, and they might be obliged to by law anyway, but I am not certain and it could depends on local laws what their actual obligation is. I feel like I used to know this ! I think throughout Europe it’s a matter of defects appearing in the item’s reasonable lifetime being covered by retailer but I might be wrong. UK law could conceivably change after Brexit but I doubt that part of it will. You could try googling your country’s laws relating to online retail.
Meantime I think you are really asking less about the legalities and more about the practicalities and ease of the process of repair or maintenance with the brand itself, and those things could vary a bit from brand to brand. I’ve never bought
Gucci. Some designer brands could make it easier than others to get timely attention, so I’d suggest you post your question again in the threads for the particular brands you are considering buying, and/or in the general purses forum, say what part of the world you are in, to get some idea of other people’s experiences with different brands and purchasing routes.
If
Fendi is on your list: I’ve also received Fendis direct from
Fendi online with very minor anomalies (not really bad enough to call them defects) and have had them replaced immediately with no quibble (and a few months later a very nice unexpected gift and apology), but I haven’t gone back with any problems later as I haven’t had any, so I can’t help with experience there. As a general point I found I preferred dealing with
Fendi online to the London boutique (over the phone, not in person) where I got the feeling I had to struggle for attention against other customers spending more. That’s a bit of a subjective appraisal though. Other people might not have had the same experience.
As far as I am aware, the brands all stand by their products wherever they were bought, and are in most cases obliged to, but whether you have to pay for repair might depend on how long the defect has taken to appear and if it is really just normal wear and tear. A retailer might give you another route but if the defect needs assessment as to normal wear and tear they might just be liaising with the brand anyway.
In the first instance, never be afraid to send a new defective bag, even with so much as a little scratch, if it bothers you, straight back for refund or replacement with a clear statement why you are doing so. Bags are for using and I don’t mind the odd scratch but I want it to be one I made myself! In Europe this is your right (I don’t know about the rest of the world). You are paying a lot for the product and it should be perfect from the start. They usually are.
Hopefully someone else can give you real details about particular brands.
I didn't end up getting the bag back then but am starting to think about it again so thanks for the advice, I feel much more confident about the whole process!! 
