1. I can't see the design, but, glueing depends on the plastic. PETG doesn't glue well, PLA does. Important in a moment.
2. 1.5mm is not strong enough even at 100%, but it only needs to be 1.5-1.6mm at the switch itself, this isn't a thin metal plate being laser cut, you can make ribs underneath the plate and if you do this, 40% infill and 3 walls is fine. BUT... and this is a big one, if your printer is not square this can create massive problems. You will need to print one part right side up another upside down and if your printer is off by 0.03mm it means a 0.6mm deviation/skew by the time you add them together. While it pays to get it as close as possible, printing one rotated 90 degrees will help offset some of it. Under plate ribs can be 5mm tall, at this point they should just touch the pcb. Beware the printer's tolerances.
3. 3mm is good for structural parts, but I tend to prefer 4 or 5mm. 3 just comes across as too thin looks-wise and often comes across as cheap. Again, I can't see your design, so it may be fine on your design. It wasn't on mine.
4. Get brass inserts but even they may not fit in your walls. Try what I do.. just use small wood screws. Pre-fit them in far enough to grab then heat with a lighter, screw them in to create threads, leave them to cool. It works surprisingly well. I tend to use #2 or #4 screws.
PLA for keyboards is HORRIBLE(!), it's rough, scratchy, sharp and "tinny" sounding. ABS is better but I DESPISE ABS. PET feels nice and sounds actually pretty decent believe it or not. The bad part is PETG needs to be printed in one piece or use screws. The other problem is PETG shrinks, not as bad as ABS, but more than PLA. Again, not seen your board, but doing even a 65% in PETG will take a LARGE printer and some real experience. Not trying to sound conceited but it really is beyond the capabilities of most people (skill/experience and printer-wise). Frankly, even in PLA a 65% is pushing the limits of FDM printing because at 3% shrink, it's way more than enough to cause lifting and of course it's too big for most printers anyhow. And ABS, PC and more, even an ergo design is a stretch. There is ways to work around this issue, but again, this comes down to experience and most people simply never have. I keep meaning to write an article about it but just haven't had a chance, the link at the bottom of this post has much of it though.
Going back to #2... If you aren't doing some right side up or down, try and keep them oriented the same. Again, skew is an issue and most people simply have never checked their printer and if they have, they've never done it at extremes. Being off 0.1mm at 100mm is perfectly fine for most things. Being off 0.1mm at 100 scaled out to 400mm now suddenly means 0.4mm skew and when flipped it becomes nearly a full mm. It may not impact your holes too bad but your outer walls will be off by that much and that is painfully obvious. This is another reason printing parts on one machine may not fit parts from another, which is surprisingly difficult.
I would highly take a look at my 65% build, tons more printed keyboard tips there.
https://geekhack.org/index.php?topic=109345.0