We do not put flowers on the altar during holy week or the midweek services during Lent. Does this include Ash Wednesday? What about the regular Sundays during Lent?
Your questions, of course, concern matters of adiaphora—issues that God, in the Bible, has not addressed specifically. Because of this, opinions and practices will vary.
Some people, desirous of promoting austerity and solemnity in worship settings, speak against the use of altar flowers during the times you mentioned. Other people, advocating Christian freedom, contend that regular and judicious use of altar flowers is a way of beautifying God’s house of worship.
All I can suggest is that congregations be ready to explain their practices. For example, because the Sundays of Lent are not counted in the 40 days of the Lenten season, some churches use altar flowers on those Sundays but not during the midweek services of Lent. A congregation with such a practice would be able to explain why they do what they do.
One of the principles of Christian worship (and the Christian life) is that we glorify God in our actions (1 Corinthians 10:31). To me, your questions illustrate a desire to do just that.