Just some thoughts on these:
If Jesus said in the Lord's Prayer: "let your will be done on earth as it is in heaven", how come JWs say that the will in heaven wasn't done until 1914?
This question was dismissed with the remark that Jesus was just telling a prophecy.
The first part of the Lord's Prayer is thought to be distilled from a prayer (the Qaddish prayer) already current in synagogues:
Exalted and hallowed be his great name
in the world which he created according to his will.May he let his kingdom rule
in your lifetime and in the days and in the lifetime of
the whole house of Israel, speedily and soon.
Praised be his great name from eternity to eternity.
The NICNT Matthew commentary has this to say about the opening petitions of the prayer: "In the light of the clear sense of [future] expectation in the Qaddish, many interpreters have argued for a similar orientation not only in the opening clauses of the Lord's Prayer but in the prayer as a whole. ... In fact, as we shall see, even vv. 9b-10 deal with matters which should be the constant concern of disciples in the present as well as with a view to the future: they desire to see God's name reverenced, his rule established, and his will done in the world as it is. While the synagogue prayer was necessarily forward-looking, for Jesus and his disciples the kingdom of God has already been announced and is working its way into the world through Jesus' ministry. In the light of that perspective, every clause of that prayer has an immediate relevance to the present situation and concerns of those who are praying." (Qaddish and quote from pp.243-4)
So part of the problem you expressed may be seen as the WT attempt to focus entirely on some prophetic aspect.
Concerning the "as in heaven, so also on earth" aspect, the NICNT (p.247) comments: "The 'aready-not yet' tension is here more explicit, as the situations in heaven (where God's kingship has been eternally honored) and on earth (where it is yet to be fully acknowledged) are compared. The time must come when God's human creatures join his angelic forces in honoring and serving their king."
A person might argue that demons were still present in the heavens as an opposing force. But that might be trying to read more detail into the prayer than Jesus intended.
Between the beginning of time and now, a lot of "faithful" people have lived and then died. Will they all fit on earth?
Dismissed as "we'll see".
Again, WT teaching obscures the possibilities. The WT teaches that only Jesus and the 144,000 are part of Abraham's seed (which is paralled by Paul with the seed of the woman, and being in the New Covenant) in Galatians chapters 3 and 4. (This is one of the reasons Randy, aka Dogpatch, reccomends reading Romans and Galatians w/o WT blinders on.) Throughout Galatians 3 and 4, Paul links these promises to those who put faith in Jesus, not to a few selected only by God's choice.
The original promise to Abraham was that his seed would become like the stars of the heavens and like the grains of sand on the seashore for multitude. The WT attempts to harmonize this with their interpretation by referring to the 'stars and sand' figure as "an unknown number," later revealed to be 144,001. But if you refernce the many verses that use the 'stars and sand' metaphor, the context always compares it with a "multitude" and contrasts it with "a few."
If you take God at his word in his promise to Abraham, it would, at some point require expansion beyond Earth, and it wouldn't necessarily ever require an end to children. Perhaps this is why the Bible never specifically refers to 'life on a paradise Earth' as a specific promise. It does imply what God's purpose for the Earth and humans was/is. But it doesn't specifically say that all human futures are based there. For the 1000 year reign, it would seem that humans would be on Earth. But after that is simply left uncommented on.
I admit that the above is based on little more than accepting God's promise made to Abraham and comparing it with what the Bible doesn't say about the future of humans. Any who want to brush it aside as speculative are welcome. But it does provide a possible answer to several of the points you brought up.
If the earth isn't already full when everyone is ressurected, people will have babies until the earth's capacity is reached, and then what? People will surely not have the desire to have babies anymore, due to God being in control.
So when the earth is filled, then what happens? The same group of people will forever live on the earth, with no one else getting a chance to see the light of day?
When exactly does this "1000-year rule" take place? Since 1914, or at a later date?
Compare Revelation 20:1 - 7. Satan and the demons are abyssed. (vv.1 - 3) Those who rule with the Christ do so for 1000 years. (vv.4 - 6) After the 1000 years is ended Satan is released. (vs.7) Satan is abyssed following the War described in chapter 19:11 - 21. That should narrow down the answer.
How can Matthew 5:3, 5 be referring to two groups of people when both should be sharing the same qualities? Shouldn't the "other sheep" and the "little flock" share the qualities of being "conscious their spiritual need" and being mild-tempered?
Matthew 5:3 says nothing about "two groups" of people. It presents the lot of all those described. It's the WT that has redifined the terms "little flock" and "other sheep." So the implication of your question is right. A thinking person rightly says; "How can this be?"