A:AnswerYou should be able to sync gmail on all four of the devices that you currently own. However, the samsung S3 tablet which runs off of your gmail account would probably be more user friendly than the kindle fire in doing so.
A:AnswerThe S3 has larger screen with a higher resolution that can be seen from nearly 180 degree (OLED based) the tab A screen is much lower resolution and does not offer the off angle viewing of the S3. The S3 has a faster processor and is slightly thinner, the Tab A is probably more "rugged" (better for kids if you don't get a case). The S3 has a newer OS and is likely to stay up to date, while the Tab A is unlikely to get an upgrade. The S3 starts with twice as much memory and has a faster graphics processor comes with a stylus and costs more than twice as much. For me the screen quality makes all the difference, but I'm not very price sensitive. Decisions, decisions.
A:AnswerAndroid Play Store.
Yes you can install the DirecTV App on it.
Its possible you could replace a Windows 7 system with this, it just depends on what you do with your Windows based system. Nearly all tablets can do word processing now even with Microsoft Office (Office 365) but to be able to print will require your printer to have wireless printing ability.
A:AnswerI don't have an cell service on my tablet. I do have the mobile hotspot service on my phone. So when I am away from home, I turn on my hotspot service on my phone and connect my tablet to it, via Wi-Fi. Check with Metro PCS and your model phone to see if they can support Mobile hotspot service.