Ans. I myself have ordered three laptops over the years from Lenovo.
The first time I got my first Legion in 2022 from Lenovo I was scared shitless just like you are.

Which site is this? No one talks about it. Everyone either buys offline or buys from amazon/flipkart. So, I decided to risk whatever little money I had and bought the laptop from there and the channel sort of started with that.
Since then I made many videos about my buying expereince from Lenovo and helped other via DMs to make them buy their gaming laptops also.
So, in short, yes, it's safe to buy from Lenovo.com but a few things to be aware of:

• It's no open box delivery. The package will be delivered to you in proper packaging.If you see any sort of tampering from anywhere(local delivery scammers do this), refuse the delivery right away.
  And regardless of if you refused the delivery or not, let lenovo know right away. They want a confirmation from you in under 24hrs.
• It's very very important to check the laptop for any defects right away. Don't postpone is just beacuse you're out of station. Get someone, even a non-tech person to try to do this as this is very very important.
  You need to test it right away.
• There was one person who was outstation and the laptop's box had some sort of damage and when he checked it days later the display was damaged.
  If this happens to you (and you didn't tell Lenovo right away)and didn't buy ADP, DON'T CONTACT LENOVO, but instead get ADP immediately wait for the ADP to show up(takes about a month), and then contact Lenovo saying you accidentally broke it stating a date just after ADP.
• If however, you did buy ADP like an absolute chad you are, just wait for it to activate(for saftey, you can contact them ealier but the brain-dead employees won't understand that you already got ADP and it's Lenovo's fault that it hasn't yet been
activated and not yours) and then tell them you want to claim ADP.
• Again, it all can be avoided if you just check it right away.

Ans. The normal stuff about delivery and checking and all applies like above but let's talk just about customized laptops in particular. ..... to write more...

Ans.

• 23 days is the average time it takes for a laptop to deliver regardless of what it says on the website and how many mails you get.
  However, during a new laptop launch or sale time(basically when they get a lotS of orders at once), the delivery can take upto 40 days.
• Till now I have not seen a longer period than that.
• But assume the given date on the site and add a few weeks on that as that's the shipping start date as your delivery time.
• to write more.....

Ans. Yes, you can upgrade by yourself. Lenovo doens't add any stickers of "Warranty Void" thing. ANd you can do it by yourself but careful not to damage any components internally especially on laptops that have extra RGB on the outside because you will have to claim ADP on it. Also, read the hardware manual of your laptop properly as it's very much detailed.

• If you call and ask any Lenovo agent or service centre people, they will scare you into making sure you do it from themselves so that they can sell you expensive SSD, RAM or just charge you lots of money from it.
• Just don't sell your old RAM/SSD until the life of the laptop. Lenovo can give you full refund on the machine if something goes horribly wrong even at the very last of the warranty period(3 years+1 year which you buy later).
• Note about upgrading SSDs: Some large capacity SSDs have chips on both sides and on some laptops their are screw standoffs(screw female that is actually a male on the outside basically LGBTQ screw that you can remove) that you must remove so that the SSD doesn't bend in the middle.
  On very few laptops there are something else on the laptop that obstructs your way.
  Once installed, you can use their own software if you plan to move the whole OS to the new SSD but I recommend moving the data only to the new SSD as that's yours.
  If you upgrade both SSDs, don't sell the older SSD until you have warranty.
  I already have some SSD recommendations here.
• Note about upgrading RAM: Just know the basic like DDR5 can only accept DDR5 RAM and you a higher RAM will automatically run at the maximum speed the CPU suggests and more which I promise to go over here along with some recommendations.
  Don't sell the old RAM until you have warranty.

Ans. No, it's written in their T&C and also in every brands T&C that the maximum ADP you can get for the discounted price is three years. After you have the laptop and normal three years ADP has activated, you can buy one year more extended but it will be more than what you paid for the three years in itself.

Ans. Check IWS FAQ and this if you already got your laptop.
But in general, it's always best to call lenovo before hand to know if the laptop you are buying in India or outside and moving to some other palce has international warranty or not.
A few Legions did have international warranty and many expensive Thinkpads but instead of assuming the same, please call the brand Lenovo as the things keeps changing all the time.
Note: I will add some real examples if someone actually does this after contacting me.

Examples

Yes, but the bonus amounts keep changing. Please check this link to know the current rate.
Lenovo does it via Cashify and this might be a bit weird but there is no sort of indication that you will get bonus cashback. You just have to make sure same contact details are used for cashify as it was used for ordering from lenovo.
I know it's so fucking dumb but it is what it is.
This video explains better:

It is not a discount.
It gets credited to the credit card bank account. The video will explain in more details.
There is no visible confirmation that you will get the credit card cashback but as long as you pay in full in a credit card and use it once during an offer you will get the credit card cashback as per T&C usually under 45 days though as per T&C it can take upto three months.
These videos will explain more: