Hi there!
As far as reliability transferring from a Spectrum to QL is concerned, the options available to improve this depend upon a number of factors and your willingness/time to tinker... Here's what I've found so far that may help guide you:
The primary issue is a very subtle bug in (both versions of) the Int-1 Shadow ROM. Due to the clever way the Int-1 ULA manages the timing of the START Bit when receiving packets, the issue doesn't seem to bother Spectrum to Spectrum comms, but only when the QL is receiving packets from the Spectrum (the QL uses an entirely SW method to detect the START Bit, with no hardware assistance from the QL's 8302 ULA)
Due to the faster CPU available to (S)GC equipped QLs, or the QXL, you can already get fairly reliable comms with the Spectrum, though the root cause is still present and does break the odd packet transfer - most users of those platforms haven't noticed it yet
A. The best fix here is to update the Int-1 ROM with a patch that I have developed to address the bug (plus a couple of other less impactful bugs), but that is tedious to achieve - to put it mildly.
B. With the TK2 ROM in place on your BBQL, there are a couple of timing constants that can be adjusted that go 75% of the way to allow proper receipt of packets from an unmod-ded Int-1/Spectrum. If you can tell me the TK2 ROM version you use, I can work out and share with you which bytes need patching, BUT you'd then need to burn a fresh TK ROM image AND the patching tends to break QL comms with other QLs (unless you also adjust them in a similar way.) Technically, a similar patching could be made to QDOS itself if you do not have TK2, but the way the Net code is designed in QDOS itself makes it a little less straightforward to tweak than in TK2. Also, I'd argue that any actual productive work with a QL requires at least TK2 - and that is especially true with Networking.
C. The inconsistent timing of Iss-5 QLs when accessing the 8302 ULA further exacerbate the issue, though this factor is less impactful. If you remain unsure whether or not your US QL is really an Iss-5, the thing to look for is whether you have a 20-pin HAL IC installed next to the 8302 ULA, or just a couple of 7400 series ICs. The HAL is part of how Sinclair 'moved' the 8302 from inside the contended video-memory bus direct on to the CPU bus, thus avoiding the access-time inconsistency that can otherwise upset the network bit-banging code.
Happy to dive in more deeply once you decide how much time you're willing to devote to the project and which of the above two options is more accessible to you...
All that said, you might just be lucky and find it works fine without any of the above (and I'd be REALLY interested to hear about it!)
As for your question about the IPC/8049 - no, it has no connection to either MDV or NET handling, and only manages KB, Sound and SERial (receive.)