This is a very confusing situation, but a few thoughts...

Any tax liability your mom had should have been paid from her estate. Not by you.

Sounds like the administrator of the estate didn't uphold their fiduciary duty. There may be legal options here and you should review the will. If you do not have a copy, a copy can be obtained from the probate court clerk in the applicable jurisdiction. Assuming it was correctly administered.

Life insurance pays out to designated beneficiaries, typically outside of an estate. Your proceeds should have come directly from the insurance firm, not some shady let's split things at the bank trip.

Considering this mostly happened a long time ago, there is likely statute if limitations in play or soon to be which stand to prohibit pursuing these issues further. Ultimately a topic to discuss with a lawyer.

When it comes to financial matters, you clearly can not trust your family. You should assume any further inquiries money wise or related to the estate from your family is an attempt to defraud you.

High chance your identity has been compromised by a family member. Please review your free credit reports and establish credit freezes with the three credit bureaus.

Recommend organizing all supporting documents related to this. Especially if you epect to discuss with a lawyer. You should be able to obtain a free consultation on the case, but considering this is a very confusing situation either an attorney won't be interested or will require a sizeable retainer fee. It is distinctly possible you and a lawyer can recover no funds even if a judgement is won but you spending $5k+ on a lawyer is very likely. Look into law clinics at the local University or non profit, you may be able to gain some free legal guidance before pursuing formally.

At some point, you'll need to consider this a closed issue and move on. Granted you should never trust the applicable parties again.

Keep in mind the maintenance on an M car is amplified quite a bit. Performance everything parts that have shorter life cycles to replace. Tires go fast, brakes are very pricey, spark plugs every 10k to 15k, etc. The mechanic labor rate is amped up for an M too.

I got a used M2 comp that was still under manufacturer warranty for a little less than a year. Enjoyed it for 6 months and moved on. Whelped a lot of money, glad I scratched that itch, but honestly it wasn't worth it.

All that to say, the way I did it is a more practical approach than buying new.

Will also say you'll find a 340i isn't that much of a compromise vs. the M3.

I've owned a 135i, 128i, and M2 comp. In reflection my favorite was the 128i, it was still quite fast and great handling but it was simple more agreeable in traffic. The M2 only wants to go fast and isn't fun inching along, granted being a manual didn't help that.