"This tea is spicy, isn't it?"
What's the distinction?
Is this some logic problem that you can solve by postulating a special spice-cancelling substance in the tea, enabling you to add spices to it - hence "spiced" - while the spice-cancelling substance ensures that the tea nevertheless remains un-spicy?
Spicy means hot. As in "don't eat the whole chili, it's spicy". Spiced means contains spices (or, I guess, alcohol). As in "I spiced the punch" or "have you tried this mulled wine? It's spiced very nicely."
Spice-y by the way would mean "in the way of a spice" or "sort of a spice".
There are many spices (typically defined as dried plant parts used to add flavor to things) that are not spicy (containing capsaicin or otherwise very "hot" to the taste).
While you can add ginger or cinnamon in high enough concentrations to make them spicy, I have many spiced teas with cloves and cinnamon and perhaps cardamom that are just pleasantly flavored, not causing any sense of burning at all.
If you used just ですよ it would be something like "Duude, this tea is spicy!" (as in like, introducing information that the hearer doesn't know yet). If you use ですね it sounds more like "wow, this tea is spicy, isn't it?(therefore you're both acquainted with the tea's flavor and you're able to give out an agreement or disagreement answer).
Overall, よ would be for emphasis on new information that the hearer might not know
Either could work in different situations.
は marks known/understood information that provides context for the statement you are about to make. This stresses what comes after it as new information.
が marks new important information. This stresses what comes before it.
With は "this tea" is known contextual information and "it is spicy, isn't it?" is being stressed. Perhaps you are both drinking tea, you think it is spicy and ask if the other person agrees. It being spicy is the important information.
With が it would emphasize "This tea" as being new and important, like "THIS TEA (is the one that is) spicy, isn't it?" Here you might have multiple different teas in front of you and you want to point out the specific one that is spicy. "This one is"
The scenario with は feels a bit more common/normal, so that's the one the contributors went with. が isn't grammatically wrong but it changes the nuance a bit to only really be usable in very specific situations.
First of all な is na. Ne is ね.
Secondly, if you just used か, the sentence would be more like "Is this tea spicy?" In contrast, with ね, it's more of "This tea is spicy, right?" The first is just a general question that you'd ask without having even drunk the tea yet. The second is something that you'd say after you've already drank the tea, formed your own opinion on it, and wanted to see if others shared that opinion.
これ is a pronoun "This one/This thing" and replaces the noun it is referring to
これは辛いお茶です [This (one)] = [Spicy tea]
この is a pre-noun adjective, it directly modifies a noun
このお茶は辛いです [This tea] = [Spicy]
You can think of この as a contraction of これ 'this' and link particle の "the tea (of this space)"
を marks the direct object of a transitive verb, the thing that a verb is acting on. There is no action in this sentence though, it is just describing a state of being, so there is no direct object.
寿司を食べます - I eat sushi (Sushi is the object of eat)
水を飲みます - I drink water (Water is the object of drink)
本を読みます - I read a book (Book is the object of read)
英語を話します - I speak English (English is the object of speak)
このお茶はからいです - This tea is spicy - Tea is the subject described as spicy. You cannot do the action of "spicy" to tea.