Search:

"Товарка" is just a very unformal word for female friend. Very-very unformal. But nothing bad about it.

Only in a very elevated language, and never in the original meaning (as "brother in arms").

Yep! When you hear "товарищ" novadays it's mostly ironical or in the very straight serious meaning, for example "боевой товарищ" - companion in arms.