"Because I am not good at swimming, I practice a lot."
I agree. It's confusing because some of the sentences say "I'm not good at", but want you to use "I'm bad at" for the translation, but then won't accept this for other sentences and want the exact translation instead and there's no way of knowing which type of translation will be accepted for the sentence.
It's possible, but it's a polite way to talk. From YesAsia about kara desu vs. desu kara:
1) からです （KARA DESU) and ですから （DESU KARA） are interchangeable.
If someone asks you "Why you are tired?", you can say;
A) あついからです。(ATSUI KARA DESU) = Because it is hot. Formal
B) あついですから。（ATSUI DESU KARA) = Because it is hot. Formal
C) あついから。 (ATSUI KARA) = Because it is hot. Informal
You can say either A or B in a formal situation. You can use C when you want to talk casually.
But when you connect the sentences ("the reason" and "the result") together, only the pattern B or C should be used.
B) あついですから、しんどいです。（ATSUI DESU KARA, SHINDOI DESU) = I am tired because it is hot.
C) あついから、しんどいです。（ATSUI KARA, SHINDOI DESU) = I am tired because it is hot.
They are grammatically correct, but please note that the sentence B sounds too polite and it sounds more natural to make the first part of the sentence (= あつい) informal as in the sentence C. But the sentence C is also polite because です is used in the second part of the sentence （= しんどいです）. As explained in Course 2 Lesson 10, the formality of the entire statement is decided by the verb in sentence 2 .
So if you just say あついから、しんどい。(ATSUI KARA SHINDOI), it sounds casual because there is no です at the end of the sentence.
Because 下手 is a na-adjective, you need to say 下手だから for a grammatically correct sentence.
From KawaKawa Learning Studio:
When placed directly after a verb or i-adjective, から kara is “because.” It can also be used as “because” with na-adjectives and nouns, but only if paired with です desu (or だ da, the short form/casual version of です desu)
Kawa-chan ga suki da kara, issho ni benkyou shitai desu.
I like Kawa-chan, so I want to study together.
No, it's not possible to put を after a verb like that. You usually add する to make a noun into a verb, so there's no need to do so with a verb. You can say 水泳をするのが (suiei o suru no ga), where 水泳 (suiei) is a noun meaning "swimming", but I don't think native speakers would normally say it that way.