Would freezing affect the flavour of mint tea

Have you tried using one green tea bag and one bag of herbal tea? There are many herbal tea sampler packs that you could get to try several flavors of tea. Since you are making iced tea, I would also recommend adding chopped fruit once the tea has chilled - I would imagine some crisp apples or even a fresh melon would add a really refreshing profile. You can even let the fruits steep in the chilled liquid, like a tea-sangria.

Another option is to create a flavored simple syrup in order to sweeten your iced tea. It's always better to use a simple syrup in a cold liquid than any kind of 'dry' sugar. Simple syrup is easy to make on the stove or in the microwave, and just add some flavors you might like while heating it. I wouldn't recommend vanilla for green tea (gut reaction), but maybe lavender, orange or cardamom?

That recipe actually says there are two ways you can prepare this:

There are two ways to do this: throw just about everything in a pot of water and simmer, then add the milk, tea and sweetener and brew for a few minutes; or, let the spices soak overnight in the water then simmer in the morning with your tea leaves and milk.

(emphasis mine)

Have you tried the latter? That way you still only have to heat things up once.

I'd be hesitant to boil milk with spices and then store it cold, reheating again. If you're using dairy milk, that is. If you're using anything other than non-fat (skim) milk, you'll get a "skin" on the top of the milk when you're boiling it - that's one of the classic signs of masala chai for me. But anyway, it becomes substantially less appetizing if it has time to cool into globules and is then reheated again. I'm not sure what problems, if any, you'd encounter using non-dairy milk like almond or soy.

You could dump the pre-ground spices into your brewed tea with milk and sugar, but the flavor profile will be a little different. Boiling the spices with the milk and tea gives everything time to meld nicely and provide a really smooth flavor blend. But I don't see any especially compelling reason that you couldn't do it.

I would like to point out, though, that boiling enough water for a travel mug's worth of tea doesn't take any more time to do on the stove than it does in my electric kettle. Adding milk shouldn't make too much difference; I'm willing to bet that after you get familiar with the routine, you can follow this recipe and still have it take 5-6 minutes. :)

Purists will insist that the spices must be boiled with the water, but those are the same people who would scoff at you adding cold milk to your tea in the first place. There's no reason why you can't, other than it might cool down the rest of your tea.