No time to read all posts so hope this is useful.
When around 12 or more years ago now - we first were asked to repair a Boxster with IMS failure - we could not obtain gaskets, seals etc (let alone the assembly tool) so had to supply an engine from a crashed car to help the customer (and this is why we know we were definitely one of the first to rebuild them from the Independent sector).
You can fit the first three rods in a conventional way but then if you fit the second half casing you cannot get access to fit the rod ends from the other side - so a new method was needed.
When eventually spares supply enabled us to rebuild an engine we heard on the grapevine that some main agents had experienced trouble with the official circlip assembly tool and some of their rebuilt engines didn't last very long before a similar failure - which frightened me that the tool was not reliable - so being a bit of a belt and braces man - we invented a way to assemble the engine differently by fitting long 6mm studding to each corner (with a wing nut and washer) for the second half case to sit on - so we could gradually lower the second half crankcase while fitting a purpose made ring clamp from underneath between the cases using an angled piston ring fitting clamping tool. This way we knew the circlips were already in place.
We had to rotate the crankshaft while doing so in a sequence with angles and piston heights following a pattern until the last piston being the centre one and all the ring sets were in the bores and the last ring clamp tool (spring steel) was withdrawn by pulling it out as it twisted - leaving space to seal the faces before closing them.
This worked perfectly but was time consuming and awkward and as numbers increased it put pressure on throughput - so eventually we obtained the official tool but made sure we used a camera to inspect the final positioning fit of each circlip.
We have since found that tool 100% reliable and easy to use - but perhaps in the early days a few mistakes were made elsewhere and we still insist that every one is checked with the camera (although this has never revealed one fitted incorrectly or having fallen out (which I agree is the most likely problem that could occur).
Other non standard pistons without the recessed larger diameter next to the circlip groove for the tool to fit in could also be a problem. When using some development pistons for various tests we have re-machined that area of the piston ourselves to allow the tool to fit although they really benefit from some extra material for the increased diameter location.
So - YES - there is a way to fit with circlips in place and without risking a fitting tool but the fitting tool can be perfectly reliable but always worth checking afterwards.
Our time-lapse video (available on our web site
www.hartech.org) might help those doing it themselves as it shows the complete strip and rebuild speeded up. I have read that someone managed to slow it down - but you can always pause it, rewind etc to follow some sequences. This might also help when refitting all the auxiliaries as it is easy until you are used to it to get something out of sequence and find you have to take things off again to make it all fit.
I am not bothered if this helps someone who cannot afford a specialist like us to do the job for them although I do recommend that if they can save the cost of the full strip and rebuild by doing some themselves - they still consider sending us the cases to have the best type of cylinder replacement carried out and a few other important mods at the same time.
The cylinder blocks are the same height to the head face from the centreline of the crankcases in all models from the 2.5 Boxster to the 3.8 997. In this time the stroke has been increased twice so later engines have the gudgeon pin reaching a lower point in the cylinder at BDC than earlier examples and can therefore be more risky if the circlips are no reliably in place.
Good luck.
Baz