A 26A EVSE would require a circuit rated at 40A, but it has a plug rated at 30A. You cannot install a combination of breaker/outlet that would meet code for this device.
The electrician is suggesting a new circuit, which is becoming sort of the the EVSE standard (likely suggesting 50A to "future proof" it). In all fairness, the added cost for the heavier wire is likely to add very little to the overall cost. I see Amazon has 32A versions of the Mustart for the same (or lower) price. If you have a new circuit installed, might as well match the EVSE to the cars capabilities.
And yes, any 240V EVSE will allow the car the precondition while charging, but anything less than a 32A version is likely to use some of your battery range (mine draws 32A when preconditioning starts, and if you are supplying less than that the battery will probably be used to make up the deficit). I understand you can even precondition on 120V once charging is complete, but that is likely also at the expense of EV range.
Code is an interesting topic. There are codes obviously for the installation of wiring and circuits. And there are codes (I assume) for hardwiring devices into the home's electrical system. But to my knowledge there are not building codes related to what you can plug into an outlet, or am I wrong about that? Are there codes that say you cannot plug more devices into a circuit that exceeds the circuit capacity? I wouldn't think so, I can have three space heaters plugged in on the same circuit as long as I only use one at a time, and not violate code as far as I know. In fact even turning them all on at the same time, while dumb, does not violate building code as far as a I know. To be sure I am not an expert on national or local electrical codes, I'm just saying code seems to be focused on installations, not what appliances are plugged into outlets, for that we have safety guidelines. But if you have examples otherwise then I will stand corrected.
To make it clear I am absolutely NOT advocating unsafe practices I am only questioning what is legal and what is simply good judgement. If my presumption is correct about what is code and what is guideline, it comes down to homeowners using good judgement when plugging things into outlets. I mentioned not using adapters, again no code violation that I know of but in my opinion that is something to be avoided just for safety. In the OP's current situation, it is contrary to accepted safety guidelines to continuously use 26 amps on a 30 amp circuit, even if that circuit is dedicated, and even if no codes are being violated.
It seems to me that switching to 8 ga wire would satisfy the spirit even if not the letter of the guideline. Or is the spirit of the 80% guideline not just about the wiring but also about the outlet and the circuit breaker? I.e. is it not safe to run more than 80% capacity through the outlet and circuit breaker even if the wiring can handle it? If that is a concern, or is an unknown, then yes there is no way to make the current situation fully safe, and a new EVSE will be required. In that case the lowest cost solution would seem to be simply getting a different low cost EVSE that works with the existing 10-30 outlet. Changing the outlet to something else would be an unnecessary cost and would still not eliminate having to get a new EVSE.
To summarize more simply in case I wasn't able to be clear in my above conjectures:
Option 1 - use existing 26 amp EVSE on 10-30 circuit. Might be safe, might not be. Might not violate any codes but goes against accepted guidelines, thus without expert determination it should be assumed to be not safe.
Option 2 - use existing 26 amp EVSE on 10-30 circuit after upgrading wire from 10 ga to 8 ga. Should be safer than option 1, but unless the 80% guideline is fully understood it still may not be fully safe. And due to the cost of new wire (depending on length) and electricians time it likely would be no cheaper than simply getting a different EVSE (option 3). But it still might be a preferred solution if the OP likes the features of the existing EVSE and doesn't want to get a different one.
Option 3 - Replace the 26 amp 10-30 EVSE with a 16 amp 10-30 EVSE (or 24 amp if available). Lowest cost solution that does not have safety questions. No electrician needed.
Option 4 - Upgrade circuit to 40 or 50 amp and get new EVSE that has the matching plug. Would be safe but much more costly than option 3 since professional installation is required. Little or no advantage to Clarity, just some potential future proofing.