Learn English – Correct use of ‘at all’ after a verb

"There are myriad" and "there are a myriad" are both correct, and in fact NGrams shows that both are far more common than the "there is" and "there's" variations.

Some weird logic has been used by some people here to suggest that "myriad" has to be singular, even though everyone would say "There are a thousand".

(In British English, collective nouns are frequently treated as plural, but even American English regularly treats numerals and expressions like "a lot" as plural.)

"There are myriad combinations" (where "myriad" is an adjective) would probably be commoner than "there are a myriad of combinations", but both are correct.

"There is a myriad of combinations", with singular agreement, should probably be regarded as correct, too. But even if it weren't, "There's" would still be acceptable colloquially, in the same way that "There's hundreds" or "There's five" is generally accepted colloquially, even though formally it should be "There are hundreds" and "There are five". ("There're" is not often seen written down, though some people say it.)

There is only one verb, is:

All I watch is the weather.

In this case the word watch is used as a noun. It is the same with All you need is love, where love is a noun.

This is called nominalisation, where a noun phrase is generated from another word class, usually a verb. In other words, the process of nominalisation turns verbs (actions or events) into nouns (things, concepts or people).