HELP! IâÂÂve Fallen, and I canâÂÂt.....OK not really....., but I do need help!

Hello, and I thank you for your time. I am doing a major remodel of my 1957 Ranch style home. It WAS 1200 sq. ft with a bath & a half. It is now 1650 sq. ft, and weâÂÂve added another full bath which now is ready for plumbing. (I should tell you now that I have pulled Permits for all work being done. IâÂÂve already passed the Drainage (inspection and pressure test) and plan to do the same with the Supply side; No Jack-Leg work goin on here). Now we have always suffered with low water pressure (even though our Static here usu. runs over 60), but didnâÂÂt know why. All I knew was that: When 2 people crack open a tap at the same time, the pressure drops dramatically; if 3 do, it drops to nothing. Also knew that the HWH, which is at the farthest point from where the Service enters, is fed by a ýâ pipe. But, because 90% of our Supply network was in the attic under a 3-on-12-roof, I knew very little else about how it was plumbed. Well I have since built a new 7-on-12 roof over about 70% of the old roof and cut out much of the old roof deck in those areas. What IâÂÂve found is this: The þâ Service is immediately reduced down to ýâ after only (1) toilet tap. It is then milked by the following ýâ branches, in order: #1 serves (2) Lavs & (1) toilet; #2 a tub/shower; #3 is the Kitchen sink. Then it makes a 42â run toward the poor thirsty HWH... but not before having to split what water is left with the Washing machine and a Hose Bibb! Then the whole thing reverses course and heads back toward the front, supplying the Dishwasher, tub and Lavs with Hot water.
NOW, just from what little I know, there is no way IâÂÂm gonna just tap into the ýâ lines near the HWH to supply the new full bath without trying to improve my overall setup in what HAS to be considered a poor design/layout. However, some areas are still inaccessible, so a complete replumb, should that be warranted or ideal, is not an option. This system is 100% galvanized pipe. There are a couple of spots in the beginning þâ where I can cut in to be able to insert PEX adapters. Also, I am planning a complete PEX install on the new supply run

Questions:
1) What if I were to add a þâ PEX run straight to the HWH to supplement the 1/2â original supply that gets bled to death on it's way there?
2) Along the same thought lines: Would a 3/4â HOT loop back to the Front end of the house help the severe pressure drops we suffer there, especially at shower time?
3) I would like to tie in the Supply runs for the new bathroom at the ideal point and size the pipe correctly. Suggestions?

Any advice is appreciated. Will try to post an isometric sketch or pic here, but if not able, contact me and I will try to send you one. Thanks in advance, CFNet ...