Is it ok to prepare chocolate ganache or mirror glaze with milk chocolate?

It will be much sweeter.

I would try a small bit first. Since there is more milk in the milk chocolate you may have to cut back a bit on the cream.

You may be able to pipe on the cake with the milk chocolate ganache but it will not firm up as much as it would if you were using a dark chocolate.

You might be better off using a mix of semi sweet and milk, that might give you a better texture than all milk chocolate and it will not be as dark and it will be a bit sweeter.

Yes. Use more milk chocolate and less cream though, than with dark.

You can use milk chocolate if you simmer it long enough maybe 15 minutes.

Absolutely, it doesn't taste quite as strong and chocolate-y but it definitely does the job if you don't have anything else.