Can't get Alt (Option) key to work while remote controlling a Mac from Windows with VNC
Problem 1) Use RealVNC client. In my experience, alt (Option) is in the Windows Key and Command is in the alt. Check out the documentation here https://www.realvnc.com/docs/mac-keyboard-mapping.html 
There is no fix for the alt (Option) key on TightVNC.

I tried to use software on the Mac to remap / change the keys functionality such as Karabiner (previously called KeyboardRemap4Macbook), Seil and even the native system Preferences > Keyboard > Modifier Keys to change ctrl for alt or capslock for alt , but it seems that when on VNC, those configurations are ignored. It's like a different keyboard is being used and it doesn't show up in the Select keyboard menu, only the local one.

Problem 2) VNC for some reason treats your keyboard as it were an US keyboard. You'll have to change in windows, the layout of your keyboard to US international, and by doing this, almost all your keys are going to be mapped correctly in the Mac. You can set different keyboard layout for different applications.

I registered a issue on TightVNC sourceforge account about this, but the project seems to be dead, so no hope. Same problem happens on RealVNC.

For those keys that are still missing / dead a workaround is to use the Character map (while editing a text press Ctrl + alt + space-bar or at the menu select Edit > Emoji &amp; Symbols )

Alternative is to use a different remote desktop solution. If it's for personal use, go for TeamViewer. I tried and it's a lot better, with full support for the keyboard, better connection, imagem, etc,
But if it's for business / comercial use, Teamviewer's price is just ridiculous IMO. The cheapest license called 'Business' cost U$ 749.