Okay, as always, this is my understanding. 1. From this sentence "此外，学习者能依据词汇的语义环境来推断意思", I would assume that you had listed a few other ways in which 学习者能够推断词汇的意思。 If that is the case, than 尽管如此 would be better because it would include everything listed above (despite everything said above), whereas 虽然如此 would be a bit ambiguous as to what you are contrasting. 2. 竟然如此 is wrong here. 竟然如此 How could it be this... 虽然如此 Although... , Despite that..., In spite of... 既然如此 Since it had come to this..., If that's the case... 竟然如此 他竟然如此天真，那番话他也信？！ 看起来容易，但没想到竟然如此复杂。 虽然如此 虽然如此，他任然很乐观。 虽然如此，我还是很爱他。 既然如此 既然如此，那我们也没什么话好说了。 既然如此，我们分手吧。 Note the usage of 竟然如此, 虽然如此 and 既然如此。 The difference between 虽然如此 and 既然如此 at the heart of it is essentially the difference between 虽然 and 既然. ps: Here's my story. Because I seldom write in Chinese, I would often mistype 既然如此 as 竟然如此, but I know full well the distinction between them. When my friends pointed it out, I would often just laugh it off and send them an emoticon.

1）“此外，学习者能依据词汇的语义环境来推断意思。虽然如此（corrected to: 尽管如此），我自己却经历了一个很奇怪的情况” -----我认为在这个句子里虽然如此和尽管如此都是可以的，都表示转折。 竟然如此：表示惊讶，后面可以接句子也可以不接。而且通常是用来对别人说的话表示惊讶。 For example: A：我昨天晚上吃了十个面包 b：竟然如此！ 虽然如此：表示转折，后面应该要接转折的内容，不能不接句子。 For example: A:我昨天晚上吃了十个面包，虽然如此，我还可以再吃下十个。

竟然如此: 两个词是分开的 It's usually used separately as a term. It has an 'unexpected', 'surprising' undertone 竟然+[如此(形容词)] eg. 她竟然如此的美丽，如此的高贵 She is such a beauty, such elegance. 虽然如此: 通常是当作一个词来用 It's usually used as one term eg. 虽然如此，但是我还是原谅了她 Despite(...), I forgave her. Hope this helps!

竟然如此美丽。带惊讶，意料之外 虽然如此糟糕。转折，后面经常接但是