First of all, you said:
Fourthly, from what you wrote it would seem reasonable to presume that the church in question does not observe the Sabbath, i.e., the Lord's Day.
You are correct in your presumption. The church in question does not believe Christians are held to Sabbath Laws. Following is a pretty accurate description of the church's (and my) beliefs (which I realize can be debatable):

Far too much to deal with on a point-by-point basis; too little time and not enough space!

However, I will make the following points as rebuttal and then refer you to some excellent articles and a book that can be found on The Highway that are representative of what the Church throughout the ages has believed and practiced until recent times.
1) It is impossible to bifurcate the Fourth Commandment from the other Nine Commandments which all admit are moral without committing hermeneutical suicide. If the Fourth Commandment is only ceremonial then so are the other Nine. They were all given at the same time by God and written on tablets of stone to indicate their perpetuity.
2) Exodus 20 shows that the Sabbath was a
creation ordinance. Deuteronomy 5 reveals further that the Sabbath is a
redemptive ordinance given to the Church.
3) Colossians 2:16, 17 is clearly NOT dealing with the NT Sabbath which is based upon the Fourth Commandment but rather "sabbath days" or if you prefer the alternate reading "a sabbath day", i.e., the Jewish Sabbath, festivals and other ceremonies which were unique to Israel. Paul isn't warning against keeping the NT Sabbath, i.e., the Lord's Day (first day of the week) unto the Lord but rather the demand to keep all the festivals, new moons, etc., which the Judaisers taught were necessary additions to faith.
4) The Sabbath was part of the initial creation of God and was kept by those who desired after God (aka: remnant) long before this commandment was
repeated at Sinai. The same applies to all of the Ten Commandments. They were written on men's hearts. For the most thorough biblical study on the Sabbath see:
The Covenantal Sabbath5) The Lord Jesus Christ kept the Sabbath faithfully as it was intended to be kept for it behoved Him to keep all the law perfectly. In fact, He was castigated for keeping the Sabbath perfectly because He didn't abide by all the extraneous "extras" that had been added by the Pharisees. For an excellent defense for the historic view that the Sabbath is binding upon NT believers (and all men for that matter), see the 3-part series by Jonathan Edwards found here:
The Perpetuity and Change of the Sabbath. And see here:
The Foundations of the Sabbath in the Word of God, by B.B. Warfield. And lastly see here:
The Sabbath Question, by James MacGregor.
6) Romans 14:5 is irrelevant for the same reason as Col 2:16, 17. There is no mention whatsoever of the Fourth Commandment. If it is made to include the Fourth Commandment, then it contradicts what Paul wrote earlier in 7:12,14,22, 25. Paul is referring to the Ten Commandments in this entire section).
Okay... enough for now.

My main question is this (but I'm also open to hearing what anyone wants to say regarding the other matter) - when would we say someone is sinning in the matter of church attendance? When they miss one Sunday (main worship services are only on Sunday mornings) due to a cause that's not "legitimate", ie, sickness, accident or otherwise being providentially hindered? When they miss 2 Sundays? When they miss regularly or routinely but still could not be said to be "forsaking" the assembling together? Doesn't "forsake" mean to totally leave?
Simply put, when someone is consistently absent from worship, either regularly or irregularly for reasons not due to God's providence, then Church should be responsible and make contact with such an individual, in person if possible for counsel. Unless the church wants to play the Bob Jones routine and make a list of all the "do's and don'ts" which are deemed "sinful", the church really doesn't have much to stand on in order to discipline someone for not attending their meetings.
When you throw out the Sabbath, you aren't left with much to discipline someone for if they don't attend worship. What if they say, "I had baseball playoffs on Sunday"? or "I have a philosophy final on Monday and I had to study for it.", or "Some friends I haven't seen in 3 years dropped by to visit." etc., etc., ad nauseam.
A desire to worship God in the assembly of the saints is a mark of regeneration. Those who have no desire to meet with God have to question their spiritual state. It's not a good sign.
![[Linked Image]](http://www.the-highway.com/Smileys/scared.gif)
And as a "cherry on top", see what you think of this sermon:
The Fourth Commandment, by Rev. G. Van Reenen.
I hope you will observe and enjoy the Lord's Day tomorrow and that God will richly bless you as you worship Him in "spirit and truth".

In His grace,