[Avodah] Nichum avelim and zugot

[Zev Sero]

I just heard for the first time of a Sefardi custom (this is second-hand
so I don't know what kind of Sefardi) not to visit an avel an even number
of times.  If one has already been to a shiva house one should not go
again unless one plans a third visit, and similarly one should not visit
a fourth time unless a fifth time is planned, etc.   The person providing
the information didn't seem to know a reason, but it seems obvious to me
that it's related to the issue of zugot.

Questions:
1. How widespread is this minhag?   Are there other zugot-related
minhagim?

2. The gemara in Arvei Pesachim says the only number to avoid is 2,
because 3, 5, and 7 can be counted or not counted, as one chooses.
So for a 4th visit one can choose not to count the 3rd, while for a
5th visit one can choose to count the 3rd after all.  So why avoid
a 4th or 6th, etc. visit?

3. I've never quite understood that gemara.  Once we know that 3 is
optionally counted, why tell us about 5 and 7?  What point is there in
choosing not to count 5 or 7, if the whole point is to arrive at an
odd number, and one can already play with 3?

-- 
Zev Sero                      The trouble with socialism is that you
zev at sero.name                 eventually run out of other people’s money
                                                     - Margaret Thatcher