Should a LT stand at ease for a 1SG or CSM?

I think this depends on the circumstance. In semi-casual situations I think it is appropriate for a LT to Stand at Ease purely out of respect, likewise the E9 should at least stand to show respect for the commission. However, if the LT is in a "Presentation of Commands" mode, the converse may be considered appropriate and the E9 should be At Ease.
With all this said, any LT who believes he is superior to an E9 and does not demonstrate a respect for that soldiers time and experience should not be surprised to find him/herself in for a rough tour.
Life for a Commission, in my opinion, does not really get underway until you make Captain.

No. The 1SG or CSM should stand at attention when talking to the LT. That is the standard.

Once upon a time at Ft Drum (A-Co 2-14) there was a young butter bar that got assigned to us. He was fresh out of West Point and still not real sure what was what....I had a profile at the time and was working as the company clerk...one day, the CSM came down from Btn, I'm not sure who he was looking for but e ended up talking to the new Lt in the CP. I remember hearing something along the lines of, "When addressing a United States Army Office you better stand accordingly!" Then I heard a lot of "Yes Sir!...Blah Blah Blah...Yes Sir! Blah Blah Blah...Yes Sir!!!!" and the Command Sergeant Major left....About 15 minutes later I heard a LOUD "AAAATENNNNTION!!!!" as the Btn Commander entered the company area and rounded the corner. As I stood there at attention with FULL BUTT PUCKER, the Colonel made a sharp left into that new LT's office. The door shut abruptly... I heard some loud noises and some muffled grunts and groans. Then I heard what sounded like furniture being moved around. I though "WOW what an awesome Commander", He came alllll the way down from Btn to our company to help this new LT rearrange furniture in his office. It must have been some heavy stuff too because when the Colonel left he was covered in sweat!!!!! So Captain Maurelli....No, a Lt does not stand at ease for a CSM, just the opposite, HOWEVER, be ready for some rearranging.... if you know what I mean....LOL!!!!!! Climb to glory!!! Right of the Line!!!

This is an absurd question. Why would you even ask this? By law, the greenest 2LT who just lowered his arm after his commissioning ceremony ranks the oldest crustiest sergeant major in the army. That's just the way it is.

NO.

Um....no. We have customs and courtesies for a reason; to show deference to rank and to carry on our traditions. A Lieutenant outranks a CSM. I once saw a CPT who got pissed at a CSM and was a bit unprofessional in his response to whatever the topic was (no, it wasn't me) and when the BN CDR found out he had a one way conversation with the CPT about how he saw the role of the CSM.
If the 1SG or CSM has been doing his job by setting the example for the LT and developing him as a leader there will be a level of mutual respect between not just with the CSM - LT relationship but between all of the Soldiers.
Senior NCOs can make the situation better by not just assuming the LT is not competent etc. Spend some time talking to them and developing them a leaders and there will be no issues with mutual respect. When going to training don't just focus on the NCOs; spend time with the PL and get him to use you as a sounding board. Once they realize that you will steer them in the right direction you have won their trust and confidence.
Young officers should understand that in most BN Command Teams the BN CDR and CSM sit down and "rack and stack" not only the Company Commanders, but the officer that the BN CDR senior rates. Make the CSM mad and the rating could suffer.

No absolutely not. While a LT should never be put in a position where they have to give orders to a 1SG/CSM (Unless its a 1LT Commanding a Company), a complete breakdown of military bearing would occur throughout a unit if this was expected of the junior officers. I have seen 2LT's stand at parade rest for CSM out of respect and I can't say it wasn't warranted for that instance but it should not be something put into practice. CSM/1SG are leadership for the enlisted men, let the LTs stand at ease for the CPTs and up.

This man commanded the respect of his lieutenants.

According to the military I joined, a 2LT/1LT still outranks a 1SG/SGM/CSM. If anything the opposite is true a Non-Commissioned Officer of any rank should stand at attention for an officer of any rank until told to stand at ease. That being said, any young officer worth his salt should show a great deal of respect to the SEAs for their experience and knowledge.

Not in my Army......