Married after 90 days, can we still apply for adjustment of status?

If the foreign spouse entered with a visa, yes you may file the visa petition (I-1`30) and adjustment of status (I-485) concurrently. Whether you need a representative is up to you. Just make sure that (s)he is an experienced immigration lawyer and not a consultant or a notary. Your case will be handled smoothly.

Yes. You can also represent yourself in criminal court but you're much much better off with an attorney. Make sure your attorney is a real attorney and not a criminal cockroach known as an "immigration consultant". You still have to prove that your marriage is bona fide and have adequate support financially. Many attorneys offer free consultations.

Did the foreign person enter with K-1 (fiancee) visa, ESTA permit, a B-2 visa, or some other visa? It is always a good idea to have a professional help you. I always have a professional mechanic work on my car. He has the tools and knowledge that I don't have.

I am concerned by what you mean by "after 90 days." Did the foreign national spouse enter on a K-visa and you failed to marry within the required 90 days? K-visa holders who do not marry within the 90 day period are in violation of immigration law, and are in danger of being placed in removal proceedings. I suggest you consult with an immigration to evaluate the risks and the effects of this violation on your foreign national spouse. Knowing the risk may help you gauge whether you should/want to hire an attorney.