Since When? - Duration Tenses

... 'for' a length of time, 'since' a particular point or moment.
Note the Past Continuous verb form in this context.

... 'for' is the only right version of the second blank in this Question.
Answer 1 would refer to a more a more distant occasion (such as 'last time we came' or 'one wet afternoon just before last Christmas'); the front part of Answer 2 would be possible.

This is 'two steps into the past' ('By back-then, she HAD been doing ... '); and we are expressing the length of her work until that time ('for'), rather than 'since when'.
We should perhaps explain that English has two special uses for the question-phrase 'Since when?':
(1.) It can be used to ask a simple question about something that has changed: 'Since when have you been putting out the rubbish and recycling on a Thursday?'
(2.) It can be used rather more 'sharply' to express doubt, surprise or even a bit of sarcasm:
'Mum, I'm going on a camping holiday next week with my boyfriend.'
'Since when?' (i.e.: When was this decided? ... but possibly also implying, 'Why wasn't I asked about this?', or even 'What new boyfriend is this, and how long have you known him?' ~ You can see what a useful question this is for making someone pause!)

Answer 2 is much the clearest and also the most stylish. The first part is fine in the Simple Past (although the fact at the heart of it is 'still true' ~ which would qualify it for the Present Tense in many other languages, logically enough); the slightly expanded phrase and idiom on the back of the sentence are strong, characterful and useful too.
Perhaps you, too, are (or have been) a motorcyclist ('since when?') and if you are a man, you may even have 'grown your own balaclava' like the man in the Question.

Answer 1 is fine here; we do not need a Continuous tense (Answer 2), and the use of Present forms with 'since' (Answers 3 & 4) ~ while clearly understandable ~ isn't the English way of doing things, however natural it may seem to you if your own language is a Romance language (French, Spanish, Portuguese, Italian) or northern European (German, Dutch, the Scandinavian group).

Answer 2 is correct here; there were wrongly-formed verbs in Answer 1.
Again, English does NOT use Present-tense forms (simple nor continuous) in such a situation, which is why Answers 3 & 4 were wrong.

Occasionally, for effect, English might instead say ' ... since last we had ... '.
('Your son seems to have grown a lot since last we saw him.')

Answer 2 is right: we use the simple past tense for one major historical event (about a century ago), and the 'had been ... since' form for something else that was already true, by then, from further back into the past.
Similarly, 'When he saw the accident in the street last week, he knew what to do since he HAD BEEN trained to deal with it (on some previous occasion).'
We can optionally drop the 'since' or 'for' in such a context: 'She had been dead 13 years; we had lived there five weeks'.

Answer 4 is also possible here. We might prefer the simple Present form, however, as this is clearer and emphasises that the 'offer' is still active at the store.

The Past Continuous form is right, here, because you 'have been' working for some while in the past, but the work continues to go on including right now.
'For hours on end' is a useful, easy and evocative phrase, meaning more or less the same as 'hour after hour'. The same structure can be used with other, longer time-units such as weeks, months or years. It would not work so well with shorter units such as minutes ('He held his breath for minutes on end' ... hmmm! ... please don't try that at home!) or seconds.
We hope that 'since doing' this Quiz, you are happier with using these structures and expressions.