both slots are occupied therefore you can not add more. You have to replace the second one. Looks like your machine runs RAID 0 therefore you have to disable RAID 0 first.
How big is your storage now ? C: + others.
A2. That is a 64 million dollars question with NO concrete answer. If no problem due to the upgrade then no problem but if something went wrong due to the upgrade you would have to talk with HP directly to work this out. HP would treat this as case-by-case basis.
Hope this makes sense.
A3. Like HDD, SSD drives are very standard now, in your case you can check the existing one from Device Manager.
A4. Normally 1 year
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