I booked my MIL SFO-SIN for next year through Aeroplan. Was planning to do the same for the return, but despite Krisflyer having several days available non-waitlist saver SIN-SFO, Aeroplan does not show the same availability. Is this because of routing issues? Would this be available as a call-to-book with Aeroplan, or do I just need to go through Krisflyer?

This is almost certainly because Krisflyer offers more award availability to its own members than it does to Aeroplan members. There is no harm calling in but I suspect you will have to book through Krisflyer.

If that's the case, wouldn't one still expect to see similar amounts of availability in each direction? I.E: less availability with Aeroplan both ways, rather than just one direction?

Unless you're saying they limit it by restricting entire routes.

As has been reported before, it is all a bit mixed up these days. There can be cases where AC or AS (MP) have availability on routes when KF shows waitlist or nothing or only Advantage awards. Similar the other way around where KF may have availability and AC, AS MP, LH M&M, etc. have nothing.

IME, unfortunately, it cannot be generally said any longer that KF will have more availability for its own members over other *A or Partner FFPs. If one has "mileage currency" across programs it helps to check both and see what comes up availability wise and what it costs in miles and charges etc. YMMV.

I booked a business class flight on Singapore Airlines (BKK-SIN-SFO in June 2023) thru Aeroplan for 87,000 miles. I transferred 88,000 miles from Capital One and it was instantaneous. There were no problems. I could also use the confirmation code on the SQ website for the "Book the Cook" meal selections.

Sorry for the late reply here! I've found that all of the award tickets held by ION are easy to book, either on the SQ website or in the SQ app; you simply click into the held reservation and "confirm it" by paying the miles and fees. You can also go back to ION to ticket it, but this will cost an extra ~$50 USD for using an agent to ticket.

Thanks. Good to know. Does ION SQ office still have long queues or not a problem any more post covid? Wonder if SQ ticketing @ Changi can do same hold award redemption

Not sure about Changi, but I just went to SQ ION a couple of weeks ago.
Made an appointment in advance: https://www.singaporeair.com/sia-ser...e/appointment/
No queue when I got there on a Sunday morning (but don't go too early if you make an appointment as your Q ticket doesn't come live until about 10-15 minutes before your appointment time). Was actually quite peaceful! Did not have to wait long before my number came up. The CSO told me that it gets busier after 12pm on Sundays.

Thanks FB. Good to know. However I managed an award hold for 2 weeks, which was then extended till for another 4 months on the phone, so may be worth trying on the phone first to save the hassle of going to the SQ office

Aeroplan award for SQ business is a pretty solid deal. I managed to snag 2 J seats from SIN-JFK with a lengthy layover in FRA for late Dec. For this route, premium economy costs 80k Krisflyer miles (saver - no promo) and business class using Aeroplan costs 87,500. A no-brainer if you can find space and are gonna be transferring points from credit card anyways.