In the USSR, the analog TV sets used SECAM, not PAL, so I imagine that the timings will be different between the UK Spectrums and the Leningrad.
SECAM is, like PAL just the colour encoding and on top of the basic B&W TV signal. Basic timing is therefore not touched. It's just about how colours are put on top - which is done in the modulator circuit anyway, as the digital output is RGB plus composite sync for the Leningrad (YUV for the Spectrum).
And I imagine that TTL logic is going to have a drastically different speed from a ULA anyway
Not in a way that it mattered with the Z80 and RAM/ROM beefing incredibly slow compared to TTL or ULA signal timing. For all practical purposes they are equivalents.
They may have needed to leave some features out to keep the chip count low enough to be affordable.
None that I know of. It would have been hard anyway, as the Spectrum doesn't exactly offer a lot of features at all :)) (*1) As tofro mentioned, there was no bus connector (*2), which may count as a missing feature (*3), though, more of a mechanical than an electronic one.
Confirming or disconfirming my speculations, is anything precisely known about the differences that caused incompatibilities between the Leningrad and the Sinclair machines?
Haven't heard of any (*4). Schematics are widely available. For example here. So it's easy to check the workings - maybe against the Spectrum Series 7 project?
*1 - In fact, as Tommy mentions mentioned, many SU/Russian spectrum clones got enhancements like a Kempston joystick interface. For the Leningrad this is true for second revision boards (Leningrad 2). Another, maybe even more appealing, was a breadboard area for free form interface building. Quite handy for a DIY computer. Last but not least, the RGB output (instead of YUV) did enable the use of professional CRTs. So maybe the question might be rather What Improvements Did the Leningrad Offer Compared With the Spectrum
*2 - Other SU/Russian clones, like the quite popular Baltik did feature an expansion port, even using real connectors instead of PCB-edge type.
*3 - Then again there might not have been many western expansion modules available at that time and less supply in edge connectors, as Eastern European designers usually preferred more reliable ways of plugging.
*4 - Except for slight differences in colour due different encoding effects only visible in side by side comparison.