Why is it that I am a Scorpio but your calendar shows the Sun is in Libra on my birthday?

You are refering to two different zodiacs. Please read the article Tropical & Sidereal Zodiacs

Having looked through your website I’m still unsure as to whether the more complex Antipodean Astro Calendar or the much simpler Moon Planting Guide would be the best for us – we most definitely are beginners to the wonderful world of BD and planting to celestial rhythms.

Would you be able to advise which would be the most suitable for a pair of beginners?

In essence:
The Antipodean Astro Calendar is comprehensive and provides information on Sun, Moon, planets, stars, a planting guide, long range weather forecasting and much else in its 32 pages.

The Moon Planting Primer is a bridging tool to the Astro Calendars and provides information on four lunar rhythms. It has 14 pages.

If your budget allows you to get both - they are complementary, if not I suggest the Antipodean Astro Calendar ...................... provided that you are not going to be alarmed and overwhelmed by how much information is in it. It is designed to suit beginners as well as researchers but some beginners can unrealistically expect to understand everything in it in a session - whereas, in reality, it can take years to assimilate what is offered in these calendars.

Tips - Reading the astrocalendar day by day, month by month, selecting one's preferred aspect to keenly study and then simply reading the rest of the material on the page out of interest, without trying too hard to understand it will, with the passing of time become a body of knowledge that we will begin to feel really at home with.
Taking the one step at a time approach just start off with something like moon phases which we are familiar with. There dates and times are easily determined from the calendar. Read what is written on the reference pages. Run with that and then build from there .......... when you are ready.

Our connections to the cosmos are simple and beautiful in their profundity but complex intheir intricacy.

What are "peppers"?

"Peppers" are a biodynamic term for an insect or weed inhibitor made from the ashes of a weed seed, insect pest, or animal pest and which look like pepper, hence the name.

Information on peppers can be obtained from local BD groups, bookshops selling biodynamic literature, or the Biodynamic national groups.
You could also ask questions on a BD online discussion group such as BDNOW.
There are many books on biodynamics. Perhaps you could try Culture and Horticulture by Wolf Storl which has been republished.

Can I use my Antipodean Astro Calendar in the Northern Hemisphere?
Can I use my Astro Calendar Northern Hemisphere in the Southern Hemisphere?

Yes ........ but you have to take into account what time zones are being used and appreciate that when the Sun, Moon or any planet is ascending in one hemisphere then it will be descending in the opposite hemisphere, and vice versa.

The Antipodean Astro Calendar uses Australian Eastern Standard Time.
The Astro Calendar Northern Hemisphere uses Pacific Time.

The Moon Planting Primer is designed to be used in any hemisphere.

What do the days that have little triangles (trines) with the plant parts signify?

By the Maria Thun planting method there are 2 sometimes differing influences on the plants that day, one from the Moon's position in front of the Zodiac and the other from planets which are 120 degrees apart from each other which is known as a "trine" relationship. When 2 planets are in 120 degrees apart they are always in the same sign. If you look at the Zodiac Circle you will see that all the fire signs are 120 apart as are the water, air and earth signs.

Maria Thun considers that the second influence overides the first.

Example:
The Moon is in a fire sign which is considered to be a fruit/seed day.
Two planets are in a trine relationship in Earth signs which makes it a root influence.
Maria Thun would plant root crops like carrots and beetroot.

At times the Moon and planet influences are mutually supportive.
When this is the case there is an asterisk next to the triangle that indicates that the moon and the planets are both influencing the same part of the plant

When there is a dot next to the triangle that indicates that the moon and the planets are giving different  influences to the plant