Banana Shallots would be a better substitution for onion if you want a milder flavour, try slow roasting the onions before using them with plenty of EVO and you will soften the flavour of the onions substantially.

They will grow fine in a deep tub, provided it will retain a decent amount of moisture...they will need to be watered regularly. Quarter them lengthwise over about 2/3 of their length so you can spread them out like a fan to wash out all the dirt trapped deep inside them.

Soups, gravies, sauces - lots and lots of uses...what do you want to use it with?

All beef cooks well slowly - and now that summer is coming along you can slow cook beef ribs using nothing but the sun - marinade them well, then place them on a rack in a covered roasting pan and leave in direct sunshine for 6-7 hours then sear on a BBQ/grill for a couple of minutes a side to caramelise the outside.

Oxtail has pretty distinctive flavour, so it's best to use it if the recipe calls for it - ask your butcher for a smaller quantity...