The E355 plugs in to a USB port (I have even used a iphone charger USB outlet on a 13amp socket to power the unit), so up to 5 devices that use wifi can connect to the Huawei E355 and the signal is then converted to use any data sim card in the unit to access the internet. I use PAYG Vodafone, but it is not locked to my supplier and I see no reason why you couldn't use a pay monthly sim for the job. You can even connect the tablet to the device plugged in to a USB charger in the car for a truly mobile connection - only limited by mobile supplier coverage!
Theoretically you can use it as wifi router for up to 5 devices as far as i know, to get to the internet, but to set it up just for that is not worth it..buy cheap router. If you mean to use it as wifi dongle to access wifi..you can't use it for that..
Yes, but it does need to be powered from a USB lead connected to a computer or battery pack, and it does require a pay sim to access the internet (this can be a day, month or quarterly time-limited and capacity a limited sim). There is no link between this device and your home internet connection. I find it more useful for internet connection whilst outdoors in the 'middle of nowhere' by using an Anker battery supply.