Multi-quote is no problem. To quote anything manually, you just gotta use the tags. If I were to manually quote something (not using the Reply button, I would need the opening quote tag, the content, and the closing tag. I'm going to space things out a bit so it doesn't try to quote something:
[ quote] <copy-paste the content> [ /quote]
Just remove the space after the bracket. That will generate a generic, non-attributed quote such as
The quick brown for jumps over the lazy red dog
If I want to manually quote PaterAlf, then I need to alter the first tag:
[ quote_2] <copy-paste the content> [ /quote]
Again, remove the spaces. The "_2" bit is what puts in the mini-avatar and also links it back to the original post. Note that if you put in the wrong number, it will show the wrong avatar and link to the wrong post. To find the number manually, look to the right of the avatar pic and hover your mouse over that top line. Just to the right of "Posted 3 hours ago" will appear both a link and a number. The number is the one you put after 'quote_'. If you hover over the link, you can right-click and Copy Link Location in case you want to link that post. This is often used if someone wants to link a specific post from an entirely different thread.
Now, you can quote down through two levels. If I were to manually quote JMich's reply to you along with your original post, it would look like:
[ quote_3]
[ quote_1] <the content you typed>
[ /quote]
<the content he typed in reply to you>
[ /quote]
If someone were to subsequently quote my post in which I quote JMich who first quoted you, then your content would disappear since that would be a third level of nested quotes.