If you were quite deliberate in not wanting to change g, why then change the definitions on keys that stayed put going from QWERTY to Colemak, e.g., a z w c v? They were just fine in QWERTY, or do you not believe that vim normal mode in QWERTY is quite good, indeed optimal?
I find z for undo especially pessimal for a text editor. It is much inferior to sticking with u. Did you choose z because it mnemonically refers to C-z for undo in other programs? To a vi user, that is poor consolation as he struggles to scoot his weak pinky to the south-west to effect an undo.
Moving the visual-mode key from one stay-put letter v to another stay-put letter a is equally bizarre. Not changing their behavior would have been a win on two counts: (1) The mental association of letter and meaning hasn't changed, and (2) The physical association of finger and meaning hasn't changed either.
You have also irretrievably lost ALL the text objects of vim! Things like "ib" for "everything in the nearest enclosing parens" or "ip" for "current paragraph" -- gone! These cannot be re-defined on other keys (:omap can only define motions). I don't see what the solution is, since you seem committed to remapping i and a. It is possible that you do not use text objects yourself, so don't really see a need for them. However that undercuts the general-purpose utility of the plugin.
colemak.vim suffers from trying to satisfy two goals: Only one of them is of relevance to a vim user: To have vim normal mode as usable on Colemak as on QWERTY. The other goal, to emulate the keyboard shortcuts from non-vi programs is not going to help those who value vi for vi's own merits. Unfortunately, that this second goal exists is not made clear.
I have already mentioned that colemak.vim invalidates huge swaths of the vim help text, the one crutch that every vim user, whether newbie or expert, has come to lean on. Learning colemak.vim is also no help for those who use their shell with "set -o vi" -- they still have to know true vi for that and any number of other programs that allow you to use "vi-style" keys, like Info or lynx. I know, it can't be helped, but even so, you could have made the difference and the pain minimal. Instead, colemak.vim makes rather lavish changes, and the changes aren't even always better than just sticking with the original vim mappings, even with the Colemak layout. Lavish changes that break everything are fine for individual tinkering, but they should not be offered as the canonical vim plugin for use with Colemak. The FAQ entry is misleading. This is a tougher problem than the FAQ entry with its blithe assurances about "total redesign" and "many advantages" would have one believe.
I notice you are fond of offering a "whole picture" rationale for the changes you propose. I would find that persuasive, except that I can't see what the whole picture gain is here. colemak.vim offers a very pessimal normal mode on the whole, so I don't see the point of the sacrifices entailed for each individual keymap change.