The first series started out at 8.30 on BBC2 which is kind of okay but it got shunted to 7pm on another day halfway through the series. The second series started out at 10.35 on BBC1, which is not great, and then after three episodes the Ten O'Clock News was extended, and so it ended up at 10.45 which is a ridiculously late slot for a pre-watershed family sitcom. The third at last had a good slot at 8.30 on Fridays - but was in the summer, and after the first two episodes it got taken off for two weeks for Election Question Time, and then one further episode got shunted back to BBC2 due to Wimbledon. Unlucky, maybe, but I'd say that's not great scheduling overall.
I wouldn't say the post-news slot was a bad one, Mrs Brown's Boys and Citizen Khan both successfully launched in that slot, and it is a fact that because sitcoms are so relentlessly slagged off these days the Beeb do feel the need to give them all exceptionally soft launches and start them on BBC1 in off-peak slots.
As you say, it did move from 8.30 on BBC2 to 8pm (it wasn't 7pm) mid-run, but obviously the ratings had come in at 8.30 and it wasn't doing much, and although as we know sitcoms take a bit of time for the audience to really embrace them, you can't really expect the audience to persevere with them for several episodes if they're not enjoying it. The Beeb would probably have been well within their rights to have dropped in after series one judging by the ratings, which were pretty hopeless, but they were willing to recommission it.
I enjoyed Count Arthur Strong but it was pretty esoteric viewing. There were some very funny bits in it, but also some bloody awful ones, and despite running for three series they never really managed to make any of the other characters memorable or funny enough to ensure it didn't flag massively every time Arthur wasn't on screen. The fact they did three series is impressive enough, I think.