...about preserving residency and applying citizen

PR residency obligation:

The PR residency obligation is best explained in the guide for applying for a PR card, in Appendix A. The guide is found here (this is a link).

It clearly explains the exceptions for counting time spent outside of Canada, including the Canadian employment exception. I am not sure to what extent the particular technicalities are parsed, but the intent of this is not to facilitate Canadian PRs living abroad, but rather to avoid penalizing Canadian PRs who have jobs in Canada but who are assigned to a position that requires them to work, for a time, outside Canada. In other words, while I am not sure to what extent this is scrutinized, to be safe, I would not rely on this exception unless you were living in Canada, employed in Canada by a Canadian company, and then obtained an assignment or transfer to a position abroad. Beware: one does see, in the IAD cases, that CBSA and CIC oft times issue removal orders (CBSA at a POE) or deny applications for PR travel documents (CIC at foreign based visa offices) based on findings that the PR involved did not qualify for the employment abroad by a Canadian company exception. (I suspect many, if not most of these cases, are rooted in a not-so-reputable consultant organized scheme, such as consultants who claim they can set PRs up to meet their PR residency obligation even while they are abroad, through setting up a Canadian company that employs the PR. NOT the way to go.)


Qualifying for citizenship:

Who can apply for citizenship is clearly explained at the CIC website; see this page (this is a link)

At that page, there is a link to the residency calculator . . . now being called the "citizenship calculator" by CIC; at the moment I get a message that their server is down. I know that they are in the process of updating the website this week (part of implementing some changes it appears), so maybe it has something to do with that.

When they make changes oft times the old url addresses will no longer work. The CIC website itself is, in most respects, relatively user-friendly and easy to navigate, so if a link here does not work you can usually navigate your way to the same information or form just going through the primary CIC portal (home page).

As to the residency calculation, alternatively, you can look at the "How to Calculate Residence" form (linked here).

Beware: merely being eligible to apply does not necessarily mean one is entitled to a grant of citizenship.

You allude to one of the issues: if you barely meet the residency requirement and then go live abroad while the application is pending, you are virtually guaranteed to get RQ and have your application scrutinized intensely, and if there is any doubt about having been actually, physically present in Canada > 1095 days during the four years immediately prior to the date you applied, it is likely to be a long haul and there is a significant risk of being denied. If you have direct proof of physically being located in Canada > 1095 days during the relevant time, no worry. I do not know anyone who has had such proof, since for most days of the year most of us are not doing something for which there is a definitive record of our physical presence anywhere.

See the citizenship forum here for a lot, lot more discussion of these issues.

Frankly, bottom-line, a PR should wait until they have actually permanently settled in Canada, with every intent to remain settled and residing in Canada, before applying for Citizenship.
A PR can maintain their PR status by merely being in Canada for two years out of five (this is an ongoing obligation, by the way, so even someone who was just issued a brand new PR card valid for another five years must still have been in-Canada for two out of the previous five years . . . in other words, a PR who renews the PR card does not get another five years in which to meet the two year obligation, but must be in continual compliance with the obligation). There is no rush to obtain citizenship for a PR who has not yet fully settled in Canada. Best to wait.