Thank you very much for all your useful comments regarding the
indeclinable Past Participles ending with -tvaa. It's actually a
quite difficult matter as I can see!
Since Buddhadatta doesn't give too many explanations, I did my
translations by looking at the English sentences he wrote in the
"Translate into Pali" section, although to me the use of "and" sounds
better too and doesn't really change a lot the meaning.
In Exercise 12 he almost exclusively uses "having" + past participle
form such as in :
2. Having gone to the village the brahman woman bought a hen yesterday.
5. The brothers of the girl, having played and bathed, ate rice.
In the following sentence he mixes the two forms:
3. The damsels went to the tank, and having bathed and played there,
came home.
In Exercise 13 there's a note saying "Use absolutives like gantvaa"
after the following sentence:
7. The queen's mother-in-law came here yesterday and went back today.
9. Our aunts will cook rice-gruel and drink it with woman friends.
As one can see from the last sentence, the actions described happen
indeed in the future although one is happening before the other, and
Buddhadatta still recommends the use of the absolutive.