Corksport has great guides on their website on product pages for seals, walnut blasting, intake manifold removal, etc. that you can follow. Damond does as well, and of course this forum has a ton if you read through people's build threads and the How-To section
Mazdaspeed 3 How-To | Mazdaspeeds.org, The New Era in Mazdaspeed, CX-7 & Ecoboost Forums!
A lot of this depends on how much you want to spend, or if you just want to do the basics, as well as how mechanically inclined you are. Go slow and take your time, read carefully and read multiple sources before you start and you can't go wrong (usually).
1. You can get flow-tested injectors here along with all the seals you need, you will want to get the upgraded injector seals if you are pulling it anyways - you can choose to send your injectors in or send back your cores once you swap. Graveyard is kind of a douchey company, but my injectors were fine and it's a pretty simple process which is why I used them, and they send along a flow-test sheet with the order.
Mazdaspeed 3/6 OEM Injector Cleaning – Graveyard Performance
-You can also send your injectors to somewhere like InjectorRX which many have done with good success, and then just get the injector seal kit and Overspeed/Corksport Injector seals.
Fuel Injector cleaning service, flow testing and rebuilding, Injector Rx
2. Yeah Damond is great in everything they do - you can do sealed or vented can depending on how you want to run it, vented doesn't hurt if you go for more power in the future and it isn't a huge price premium.
Mazdaspeed3 Oil Catch Can Kit Stage 2 | Damond Motorsports
3.
How to Clean Your Intake Valves | CorkSport Mazda Performance Blog - Corksport makes a tool for this, but I followed that guide and use those parts every now and then for a quick clean. Prep is the most important to prevent a lot of mess, so spend a good amount of time taping up and getting things sealed before you go to town. I found a massive black trashbag works great.
4. OEM is the safest bet when it comes to PCV, Thermostat, sensors, timing kits, etc. If you have / want to spend the money, you can get the plate too - it isn't necessary with just one catch can however.
5. Yes you can / should only go OEM. I got mine on Ebay for way cheaper than anywhere else and it is mint. You can get it from anywhere though as long as the part number matches (L3K9-13-2J0)
Mazda Genuine L3K9-13-2J0 Fuel Distributor Pressure Valve Turbo 2.3L CX7 OEM | eBay